diff --git a/lua/submode/init.lua b/lua/submode/init.lua
index fd3c0d0..f20918d 100644
--- a/lua/submode/init.lua
+++ b/lua/submode/init.lua
@@ -34,10 +34,7 @@ function M.create(name, info, register)
     -- Judge to continue process by checking `override_behavior`.
     if state.submode_to_info[name] then
         if state.submode_to_info[name].override_behavior == "error" then
-            vim.notify(string.format("submode `%s` already exists", name), vim.log.levels.ERROR, {
-                title = "submode.nvim",
-            })
-            return
+            error(string.format("submode `%s` already exists", name))
         elseif state.submode_to_info[name].override_behavior == "keep" then
             return
         end
@@ -174,10 +171,7 @@ function M.enter(name)
 
     -- Validate given submode's name.
     if not info then
-        vim.notify(string.format("submode `%s` doesn't exist", name), vim.log.levels.ERROR, {
-            title = "submode.nvim",
-        })
-        return
+        error(string.format("submode `%s` doesn't exist", name))
     end
 
     -- Validate that current mode and submode's parent mode is same
diff --git a/lua/submode/mode.lua b/lua/submode/mode.lua
index bdc4cb6..a14db2d 100644
--- a/lua/submode/mode.lua
+++ b/lua/submode/mode.lua
@@ -90,10 +90,7 @@ function M.is_parent_same(submode, name)
         ["!"] = fany { M.is_insert_mode, M.is_cmdline_mode },
         [""] = fany { M.is_normal_mode, M.is_visual_mode, M.is_o_pending_mode },
     }, function()
-        vim.notify(string.format("invalid mode `%s`", parent), vim.log.levels.ERROR, {
-            title = "submode.nvim",
-        })
-        return false
+        error(string.format("invalid mode `%s`", parent))
     end)
 end