diff --git a/.github/workflows/mypy.yml b/.github/workflows/mypy.yml
index d358f2f6..70d3180c 100644
--- a/.github/workflows/mypy.yml
+++ b/.github/workflows/mypy.yml
@@ -18,4 +18,4 @@ jobs:
           mypy --strict --show-error-codes app reportlab_mods.py > error-summary.txt
           status=$?
           cat error-summary.txt
-          [ $status -eq 0 ] || [ $(tail -n1 error-summary.txt | cut -d' ' -f2) -le 50 ]
+          [ $status -eq 0 ] || [ $(tail -n1 error-summary.txt | cut -d' ' -f2) -le 20 ]
diff --git a/app/auth.py b/app/auth.py
index 859b6454..785c26e7 100644
--- a/app/auth.py
+++ b/app/auth.py
@@ -1,3 +1,4 @@
+import functools
 from typing import Any
 
 import jwt
@@ -25,23 +26,31 @@ class JWTAuthorizationCredentials(BaseModel):
     message: str
 
 
+@functools.lru_cache
+def get_keys() -> dict[str, JWK]:
+    return {
+        jwk["kid"]: jwk
+        for jwk in JWKS.model_validate(
+            requests.get(
+                f"https://cognito-idp.{app_settings.aws_region}.amazonaws.com/"
+                f"{app_settings.cognito_pool_id}/.well-known/jwks.json"
+            ).json()
+        ).keys
+    }
+
+
+# https://docs.aws.amazon.com/cognito/latest/developerguide/amazon-cognito-user-pools-using-tokens-verifying-a-jwt.html
 class JWTAuthorization(HTTPBearer):
     """
     An extension of HTTPBearer authentication to verify JWT (JSON Web Tokens) with public keys.
     This class loads and keeps track of public keys from an external source and verifies incoming tokens.
 
     :param auto_error: If set to True, automatic error responses will be sent when request authentication fails.
-                       Default is True.
     """
 
     def __init__(self, auto_error: bool = True):
         super().__init__(auto_error=auto_error)
-        self.kid_to_jwk: dict[str, JWK] | None = None
-
-    def load_keys(self) -> None:
-        if self.kid_to_jwk is None:
-            jwks = _get_public_keys()
-            self.kid_to_jwk = {jwk["kid"]: jwk for jwk in jwks.keys}
+        self.kid_to_jwk = get_keys()
 
     def verify_jwk_token(self, jwt_credentials: JWTAuthorizationCredentials) -> bool:
         """
@@ -50,9 +59,16 @@ def verify_jwk_token(self, jwt_credentials: JWTAuthorizationCredentials) -> bool
         :param jwt_credentials: JWT credentials extracted from the request.
         :return: Returns True if the token is verified, False otherwise.
         """
-        self.load_keys()
         try:
-            public_key = self.kid_to_jwk[jwt_credentials.header["kid"]]
+            kid = jwt_credentials.header["kid"]
+
+            # "If you receive a token with the correct issuer but a different kid, Amazon Cognito might have rotated
+            # the signing key. Refresh the cache from your user pool jwks_uri endpoint."
+            if kid not in self.kid_to_jwk:
+                get_keys.cache_clear()
+                self.kid_to_jwk = get_keys()
+
+            public_key = self.kid_to_jwk[kid]
         except KeyError:
             raise HTTPException(
                 status_code=status.HTTP_403_FORBIDDEN,
@@ -64,6 +80,7 @@ def verify_jwk_token(self, jwt_credentials: JWTAuthorizationCredentials) -> bool
 
         obj = jwt.PyJWK(public_key)
         alg_obj = obj.Algorithm
+        assert alg_obj
         prepared_key = alg_obj.prepare_key(obj.key)
 
         return alg_obj.verify(msg, prepared_key, sig)
@@ -75,8 +92,6 @@ async def __call__(self, request: Request) -> JWTAuthorizationCredentials:
         :param request: Incoming request instance.
         :return: JWT credentials if the token is verified.
         """
-        self.load_keys()
-
         if credentials := await super().__call__(request):
             if not credentials.scheme == "Bearer":
                 raise HTTPException(
@@ -120,27 +135,3 @@ async def __call__(self, request: Request) -> JWTAuthorizationCredentials:
                 status_code=status.HTTP_403_FORBIDDEN,
                 detail=_("Not authenticated"),
             )
-
-
-public_keys = None
-
-
-def _get_public_keys() -> JWKS:
-    """
-    Retrieves the public keys from the well-known JWKS (JSON Web Key Set) endpoint of Cognito.
-
-    The function caches the fetched keys in a global variable `public_keys` to avoid repetitive calls
-    to the endpoint.
-
-    :return: The parsed JWKS, an object which holds a list of keys.
-    """
-    global public_keys
-    if public_keys is None:
-        public_keys = JWKS.model_validate(
-            # https://docs.aws.amazon.com/cognito/latest/developerguide/amazon-cognito-user-pools-using-tokens-verifying-a-jwt.html
-            requests.get(
-                f"https://cognito-idp.{app_settings.aws_region}.amazonaws.com/"
-                f"{app_settings.cognito_pool_id}/.well-known/jwks.json"
-            ).json()
-        )
-    return public_keys
diff --git a/app/aws.py b/app/aws.py
index 02ce7691..8e882b34 100644
--- a/app/aws.py
+++ b/app/aws.py
@@ -104,7 +104,7 @@ def respond_to_auth_challenge(
         challenge_name: literals.ChallengeNameTypeType,
         new_password: str = "",
         mfa_code: str = "",
-    ) -> type_defs.RespondToAuthChallengeResponseTypeDef | dict[str, str]:
+    ) -> type_defs.RespondToAuthChallengeResponseTypeDef:
         """
         Responds to the authentication challenge provided by Cognito.
 
@@ -156,7 +156,7 @@ def respond_to_auth_challenge(
                     Session=verify_response["Session"],
                 )
             case "SOFTWARE_TOKEN_MFA":
-                challenge_response = self.cognito.respond_to_auth_challenge(
+                return self.cognito.respond_to_auth_challenge(
                     ClientId=app_settings.cognito_client_id,
                     ChallengeName=challenge_name,
                     ChallengeResponses={
@@ -166,11 +166,6 @@ def respond_to_auth_challenge(
                     },
                     Session=session,
                 )
-
-                return {
-                    "access_token": challenge_response["AuthenticationResult"]["AccessToken"],
-                    "refresh_token": challenge_response["AuthenticationResult"]["RefreshToken"],
-                }
             case _:
                 raise HTTPException(
                     status_code=status.HTTP_501_NOT_IMPLEMENTED,
diff --git a/app/models.py b/app/models.py
index fa1c848e..e8baf5dc 100644
--- a/app/models.py
+++ b/app/models.py
@@ -86,7 +86,7 @@ def create(cls, session: Session, **data: Any) -> Self:
         return obj
 
     @classmethod
-    def create_from_object(cls, session: Session, obj: Any) -> Self:
+    def create_from_object(cls, session: Session, obj: Self) -> Self:
         """
         Insert a new instance into the database.
 
@@ -529,7 +529,7 @@ class Award(AwardBase, ActiveRecordMixin, table=True):
 
     # Relationships
     applications: list["Application"] = Relationship(back_populates="award")
-    borrower: Borrower = Relationship(back_populates="awards")
+    borrower: Borrower | None = Relationship(back_populates="awards")
 
     # Timestamps
     created_at: datetime = Field(
@@ -653,8 +653,8 @@ class ApplicationBase(SQLModel):
     archived_at: datetime | None = Field(sa_column=Column(DateTime(timezone=True)))
 
     # Relationships
-    award_id: int | None = Field(foreign_key="award.id", index=True)
-    borrower_id: int | None = Field(foreign_key="borrower.id", index=True)
+    award_id: int = Field(foreign_key="award.id", index=True)
+    borrower_id: int = Field(foreign_key="borrower.id", index=True)
     lender_id: int | None = Field(foreign_key="lender.id")
     credit_product_id: int | None = Field(foreign_key="credit_product.id")
 
@@ -678,9 +678,10 @@ class Application(ApplicationPrivate, ActiveRecordMixin, table=True):
     borrower_documents: list["BorrowerDocument"] = Relationship(back_populates="application")
     award: Award = Relationship(back_populates="applications")
     borrower: Borrower = Relationship(back_populates="applications")
-    lender: Lender | None = Relationship(back_populates="applications")
+    lender: Lender = Relationship(back_populates="applications")
     messages: list["Message"] = Relationship(back_populates="application")
     actions: list["ApplicationAction"] = Relationship(back_populates="application")
+    # no back_populates, because models.CreditProduct is used as a request and response format. :issue:`376`
     credit_product: CreditProduct = Relationship()
 
     @classmethod
@@ -946,7 +947,7 @@ class BorrowerDocument(BorrowerDocumentBase, ActiveRecordMixin, table=True):
     file: bytes
 
     # Relationships
-    application: Application | None = Relationship(back_populates="borrower_documents")
+    application: Application = Relationship(back_populates="borrower_documents")
 
 
 class Message(SQLModel, ActiveRecordMixin, table=True):
@@ -960,7 +961,7 @@ class Message(SQLModel, ActiveRecordMixin, table=True):
 
     # Relationships
     application_id: int = Field(foreign_key="application.id")
-    application: Application | None = Relationship(back_populates="messages")
+    application: Application = Relationship(back_populates="messages")
     lender_id: int | None = Field(default=None, foreign_key="lender.id")
 
     # Timestamps
@@ -1036,7 +1037,7 @@ class ApplicationAction(SQLModel, ActiveRecordMixin, table=True):
 
     # Relationships
     application_id: int = Field(foreign_key="application.id")
-    application: Application | None = Relationship(back_populates="actions")
+    application: Application = Relationship(back_populates="actions")
     user_id: int | None = Field(default=None, foreign_key="credere_user.id")
     user: User | None = Relationship(back_populates="application_actions")
 
diff --git a/app/parsers.py b/app/parsers.py
index ed131d3a..19021d3b 100644
--- a/app/parsers.py
+++ b/app/parsers.py
@@ -10,8 +10,12 @@
 class BasicUser(BaseModel):
     username: str
     name: str | None = None
-    password: str | None = None
-    temp_password: str | None = None
+    password: str
+    temp_password: str
+
+
+class ResetPassword(BaseModel):
+    username: str
 
 
 class SetupMFA(BaseModel):
diff --git a/app/routers/users.py b/app/routers/users.py
index e6a751a6..3996c829 100644
--- a/app/routers/users.py
+++ b/app/routers/users.py
@@ -71,11 +71,11 @@ def change_password(
     and handles different scenarios such as new password requirement, MFA setup, and error handling.
     """
     # This endpoint is only called for new users, to replace the generated password.
-    response = client.initiate_auth(payload.username, payload.temp_password)
-    if response["ChallengeName"] == "NEW_PASSWORD_REQUIRED":
-        response = client.respond_to_auth_challenge(
+    initiate_response = client.initiate_auth(payload.username, payload.temp_password)
+    if initiate_response["ChallengeName"] == "NEW_PASSWORD_REQUIRED":
+        respond_response = client.respond_to_auth_challenge(
             username=payload.username,
-            session=response["Session"],
+            session=initiate_response["Session"],
             challenge_name="NEW_PASSWORD_REQUIRED",
             new_password=payload.password,
         )
@@ -88,9 +88,9 @@ def change_password(
         UserAttributes=[{"Name": "email_verified", "Value": "true"}],
     )
 
-    if "ChallengeName" in response and response["ChallengeName"] == "MFA_SETUP":
+    if "ChallengeName" in respond_response and respond_response["ChallengeName"] == "MFA_SETUP":
         # https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/cognito-idp/client/associate_software_token.html
-        associate_response = client.cognito.associate_software_token(Session=response["Session"])
+        associate_response = client.cognito.associate_software_token(Session=respond_response["Session"])
 
         return serializers.ChangePasswordResponse(
             detail=_("Password changed with MFA setup required"),
@@ -189,8 +189,8 @@ def login(
 
     return serializers.LoginResponse(
         user=user,
-        access_token=mfa_login_response["access_token"],
-        refresh_token=mfa_login_response["refresh_token"],
+        access_token=mfa_login_response["AuthenticationResult"]["AccessToken"],
+        refresh_token=mfa_login_response["AuthenticationResult"]["RefreshToken"],
     )
 
 
@@ -240,7 +240,7 @@ def me(
     "/users/forgot-password",
 )
 def forgot_password(
-    payload: parsers.BasicUser,
+    payload: parsers.ResetPassword,
     client: aws.Client = Depends(dependencies.get_aws_client),
 ) -> serializers.ResponseBase:
     """
diff --git a/docs/_static/routes.csv b/docs/_static/routes.csv
index 199af383..eb23b999 100644
--- a/docs/_static/routes.csv
+++ b/docs/_static/routes.csv
@@ -5,7 +5,7 @@ PUT,/users/setup-mfa,SetupMFA,ResponseBase,SetupMFAInput,IResponse
 POST,/users/login,BasicUser,LoginResponse,LoginInput,ILoginResponse
 GET,/users/logout,,ResponseBase,,IResponse
 GET,/users/me,,UserResponse,,IUserResponse
-POST,/users/forgot-password,BasicUser,ResponseBase,ResetPasswordInput,IResponse
+POST,/users/forgot-password,ResetPassword,ResponseBase,ResetPasswordInput,IResponse
 GET,/users/{user_id},user_id,models.User,id,IUser
 GET,/users,"page, page_size, sort_field, sort_order",UserListResponse,PaginationInput,IUsersListResponse
 PUT,/users/{user_id},models.User,models.UserWithLender,UpdateUserInput,IUser
diff --git a/migrations/versions/d9b564fd6859_award_id_borrower_id_nonnullable.py b/migrations/versions/d9b564fd6859_award_id_borrower_id_nonnullable.py
new file mode 100644
index 00000000..2f34420a
--- /dev/null
+++ b/migrations/versions/d9b564fd6859_award_id_borrower_id_nonnullable.py
@@ -0,0 +1,31 @@
+"""award id borrower id nonnullable
+
+Revision ID: d9b564fd6859
+Revises: eaef8e99aee2
+Create Date: 2024-08-23 15:38:20.028504
+
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "d9b564fd6859"
+down_revision = "eaef8e99aee2"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.alter_column("application", "award_id", existing_type=sa.INTEGER(), nullable=False)
+    op.alter_column("application", "borrower_id", existing_type=sa.INTEGER(), nullable=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.alter_column("application", "borrower_id", existing_type=sa.INTEGER(), nullable=True)
+    op.alter_column("application", "award_id", existing_type=sa.INTEGER(), nullable=True)
+    # ### end Alembic commands ###
diff --git a/reportlab_mods.py b/reportlab_mods.py
index c6e347f5..f9011b03 100644
--- a/reportlab_mods.py
+++ b/reportlab_mods.py
@@ -27,7 +27,7 @@
 )
 
 width, height = A4
-styles = getSampleStyleSheet()
+styles = getSampleStyleSheet()  # type: ignore[no-untyped-call]
 styleN = styles["BodyText"]
 styleN.fontName = "GTEestiProDisplay"
 styleN.alignment = TA_LEFT
diff --git a/tests/routers/test_users.py b/tests/routers/test_users.py
index 0fd376f0..1ee1c356 100644
--- a/tests/routers/test_users.py
+++ b/tests/routers/test_users.py
@@ -65,7 +65,7 @@ def test_duplicate_user(client, admin_header, user_payload):
 
 
 def test_login_invalid_username(client):
-    response = client.post("/users/login", json={"username": "nonexistent"})
+    response = client.post("/users/login", json={"username": "nonexistent", "password": "", "temp_password": ""})
 
     assert response.status_code == status.HTTP_403_FORBIDDEN
     assert response.json() == {"detail": _("Invalid username or password")}