diff --git a/.github/workflows/R-CMD-check.yaml b/.github/workflows/R-CMD-check.yaml
index a668d3f5..d2ed986b 100644
--- a/.github/workflows/R-CMD-check.yaml
+++ b/.github/workflows/R-CMD-check.yaml
@@ -29,6 +29,8 @@ jobs:
           - {os: windows-latest, r: '3.6'}
           # use 4.1 to check with rtools40's older compiler
           - {os: windows-latest, r: '4.1'}
+          # keep this until the format specifier dust is truly settled (#524)
+          - {os: windows-latest, r: 'devel'}
 
           - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
           - {os: ubuntu-latest,   r: 'release'}
diff --git a/NEWS.md b/NEWS.md
index 7100deea..b8841309 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,7 @@
 # vroom (development version)
 
+* Internal changes requested by CRAN around format specification (#524).
+
 # vroom 1.6.4
 
 * It is now possible (again?) to read from a list of connections (@bairdj, #514).
diff --git a/src/r_utils.h b/src/r_utils.h
index fc5926fd..c09c0794 100644
--- a/src/r_utils.h
+++ b/src/r_utils.h
@@ -9,6 +9,14 @@
 #include <string>
 #include <vector>
 
+#ifndef R_PRIdXLEN_T
+#  ifdef LONG_VECTOR_SUPPORT
+#    define R_PRIdXLEN_T "td"
+#  else
+#    define R_PRIdXLEN_T "d"
+#  endif
+#endif
+
 namespace vroom {
 
 inline std::string
diff --git a/src/unicode_fopen.h b/src/unicode_fopen.h
index eac96dc7..7ad47ee3 100644
--- a/src/unicode_fopen.h
+++ b/src/unicode_fopen.h
@@ -49,7 +49,7 @@ inline FILE* unicode_fopen(const char* path, const char* mode) {
   }
   buf = (wchar_t*)R_alloc(len, sizeof(wchar_t));
   if (buf == NULL) {
-    Rf_error("Could not allocate buffer of size: %ll", len);
+    Rf_error("Could not allocate buffer of size: %zu", len);
   }
 
   MultiByteToWideChar(CP_UTF8, 0, path, -1, buf, len);
@@ -75,7 +75,7 @@ make_mmap_source(const char* file, std::error_code& error) {
   }
   buf = (wchar_t*)malloc(len * sizeof(wchar_t));
   if (buf == NULL) {
-    Rf_error("Could not allocate buffer of size: %ll", len);
+    Rf_error("Could not allocate buffer of size: %zu", len);
   }
 
   MultiByteToWideChar(CP_UTF8, 0, file, -1, buf, len);
diff --git a/src/utils.h b/src/utils.h
index 97c1a95c..62e3f43c 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -5,6 +5,7 @@
 #include <cstring>
 #include <sstream>
 #include <string>
+#include <tuple>
 
 namespace vroom {
 
diff --git a/src/vroom_big_int.h b/src/vroom_big_int.h
index eaa2bba0..ba225d21 100644
--- a/src/vroom_big_int.h
+++ b/src/vroom_big_int.h
@@ -6,6 +6,7 @@
 
 constexpr long long NA_INTEGER64 = 0x8000000000000000LL;
 
+#include "r_utils.h"
 #include "vroom.h"
 
 namespace cpp11 {
@@ -56,7 +57,7 @@ class vroom_big_int : public vroom_vec {
   static Rboolean
   Inspect(SEXP x, int, int, int, void (*)(SEXP, int, int, int)) {
     Rprintf(
-        "vroom_big_int (len=%d, materialized=%s)\n",
+        "vroom_big_int (len=%" R_PRIdXLEN_T ", materialized=%s)\n",
         Length(x),
         R_altrep_data2(x) != R_NilValue ? "T" : "F");
     return TRUE;
diff --git a/src/vroom_chr.h b/src/vroom_chr.h
index ffc90e4c..f101b3eb 100644
--- a/src/vroom_chr.h
+++ b/src/vroom_chr.h
@@ -4,6 +4,7 @@
 
 #include "altrep.h"
 
+#include "r_utils.h"
 #include "vroom_vec.h"
 
 cpp11::strings read_chr(vroom_vec_info* info);
@@ -38,7 +39,7 @@ struct vroom_chr : vroom_vec {
   static Rboolean
   Inspect(SEXP x, int, int, int, void (*)(SEXP, int, int, int)) {
     Rprintf(
-        "vroom_chr (len=%d, materialized=%s)\n",
+        "vroom_chr (len=%" R_PRIdXLEN_T ", materialized=%s)\n",
         Length(x),
         R_altrep_data2(x) != R_NilValue ? "T" : "F");
     return TRUE;
diff --git a/src/vroom_date.h b/src/vroom_date.h
index d9fcd098..c0acb807 100644
--- a/src/vroom_date.h
+++ b/src/vroom_date.h
@@ -2,6 +2,7 @@
 
 #include <cpp11/doubles.hpp>
 
+#include "r_utils.h"
 #include "vroom_dttm.h"
 
 using namespace vroom;
@@ -47,7 +48,7 @@ class vroom_date : public vroom_dttm {
   static Rboolean
   Inspect(SEXP x, int, int, int, void (*)(SEXP, int, int, int)) {
     Rprintf(
-        "vroom_date (len=%d, materialized=%s)\n",
+        "vroom_date (len=%" R_PRIdXLEN_T ", materialized=%s)\n",
         Length(x),
         R_altrep_data2(x) != R_NilValue ? "T" : "F");
     return TRUE;
diff --git a/src/vroom_dbl.h b/src/vroom_dbl.h
index c9699ef9..38f4954e 100644
--- a/src/vroom_dbl.h
+++ b/src/vroom_dbl.h
@@ -2,6 +2,7 @@
 
 #include "altrep.h"
 #include "parallel.h"
+#include "r_utils.h"
 #include "vroom_vec.h"
 
 double bsd_strtod(const char* begin, const char* end, const char decimalMark);
@@ -37,7 +38,7 @@ class vroom_dbl : public vroom_vec {
   static Rboolean
   Inspect(SEXP x, int, int, int, void (*)(SEXP, int, int, int)) {
     Rprintf(
-        "vroom_dbl (len=%d, materialized=%s)\n",
+        "vroom_dbl (len=%" R_PRIdXLEN_T ", materialized=%s)\n",
         Length(x),
         R_altrep_data2(x) != R_NilValue ? "T" : "F");
     return TRUE;
diff --git a/src/vroom_dttm.h b/src/vroom_dttm.h
index 9ab90798..6d96ec32 100644
--- a/src/vroom_dttm.h
+++ b/src/vroom_dttm.h
@@ -3,6 +3,7 @@
 #include <cpp11/doubles.hpp>
 #include <cpp11/integers.hpp>
 
+#include "r_utils.h"
 #include "vroom.h"
 #include "vroom_vec.h"
 
@@ -65,7 +66,7 @@ class vroom_dttm : public vroom_vec {
   static Rboolean
   Inspect(SEXP x, int, int, int, void (*)(SEXP, int, int, int)) {
     Rprintf(
-        "vroom_dttm (len=%d, materialized=%s)\n",
+        "vroom_dttm (len=%" R_PRIdXLEN_T ", materialized=%s)\n",
         Length(x),
         R_altrep_data2(x) != R_NilValue ? "T" : "F");
     return TRUE;
diff --git a/src/vroom_fct.h b/src/vroom_fct.h
index 14083ad6..41434b0b 100644
--- a/src/vroom_fct.h
+++ b/src/vroom_fct.h
@@ -3,6 +3,7 @@
 #include <cpp11/strings.hpp>
 
 #include "altrep.h"
+#include "r_utils.h"
 #include "vroom.h"
 #include "vroom_vec.h"
 
@@ -122,7 +123,7 @@ struct vroom_fct : vroom_vec {
   static Rboolean
   Inspect(SEXP x, int, int, int, void (*)(SEXP, int, int, int)) {
     Rprintf(
-        "vroom_factor (len=%d, materialized=%s)\n",
+        "vroom_factor (len=%" R_PRIdXLEN_T ", materialized=%s)\n",
         Length(x),
         R_altrep_data2(x) != R_NilValue ? "T" : "F");
     return TRUE;
diff --git a/src/vroom_int.h b/src/vroom_int.h
index b61a1acc..0fdb1bda 100644
--- a/src/vroom_int.h
+++ b/src/vroom_int.h
@@ -2,6 +2,7 @@
 
 #include "altrep.h"
 
+#include "r_utils.h"
 #include "vroom_vec.h"
 
 int strtoi(const char* begin, const char* end);
@@ -35,7 +36,7 @@ class vroom_int : public vroom_vec {
   static Rboolean
   Inspect(SEXP x, int, int, int, void (*)(SEXP, int, int, int)) {
     Rprintf(
-        "vroom_int (len=%d, materialized=%s)\n",
+        "vroom_int (len=%" R_PRIdXLEN_T ", materialized=%s)\n",
         Length(x),
         R_altrep_data2(x) != R_NilValue ? "T" : "F");
     return TRUE;
diff --git a/src/vroom_num.h b/src/vroom_num.h
index b21eacd5..665502fe 100644
--- a/src/vroom_num.h
+++ b/src/vroom_num.h
@@ -4,6 +4,7 @@
 
 #include "altrep.h"
 
+#include "r_utils.h"
 #include "vroom_vec.h"
 
 #include "parallel.h"
@@ -47,7 +48,7 @@ class vroom_num : public vroom_vec {
   static Rboolean
   Inspect(SEXP x, int, int, int, void (*)(SEXP, int, int, int)) {
     Rprintf(
-        "vroom_num (len=%d, materialized=%s)\n",
+        "vroom_num (len=%" R_PRIdXLEN_T ", materialized=%s)\n",
         Length(x),
         R_altrep_data2(x) != R_NilValue ? "T" : "F");
     return TRUE;
diff --git a/src/vroom_rle.h b/src/vroom_rle.h
index d47373ca..bc05e20b 100644
--- a/src/vroom_rle.h
+++ b/src/vroom_rle.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "altrep.h"
+#include "r_utils.h"
 
 #ifdef HAS_ALTREP
 
@@ -40,7 +41,7 @@ class vroom_rle {
   static Rboolean
   Inspect(SEXP x, int, int, int, void (*)(SEXP, int, int, int)) {
     Rprintf(
-        "vroom_rle (len=%d, materialized=%s)\n",
+        "vroom_rle (len=%" R_PRIdXLEN_T ", materialized=%s)\n",
         Length(x),
         R_altrep_data2(x) != R_NilValue ? "T" : "F");
     return TRUE;
diff --git a/src/vroom_time.h b/src/vroom_time.h
index 1a8d5de5..32d4e01d 100644
--- a/src/vroom_time.h
+++ b/src/vroom_time.h
@@ -2,6 +2,7 @@
 
 #include <cpp11/doubles.hpp>
 
+#include "r_utils.h"
 #include "vroom.h"
 #include "vroom_dttm.h"
 
@@ -47,7 +48,7 @@ class vroom_time : public vroom_dttm {
   static Rboolean
   Inspect(SEXP x, int, int, int, void (*)(SEXP, int, int, int)) {
     Rprintf(
-        "vroom_time (len=%d, materialized=%s)\n",
+        "vroom_time (len=%" R_PRIdXLEN_T ", materialized=%s)\n",
         Length(x),
         R_altrep_data2(x) != R_NilValue ? "T" : "F");
     return TRUE;
diff --git a/src/vroom_write.cc b/src/vroom_write.cc
index e399acee..b3624710 100644
--- a/src/vroom_write.cc
+++ b/src/vroom_write.cc
@@ -270,7 +270,7 @@ void write_buf_con(const std::vector<char>& buf, SEXP con, bool is_stdout) {
   if (is_stdout) {
     std::string out;
     std::copy(buf.begin(), buf.end(), std::back_inserter(out));
-    Rprintf("%.*s", buf.size(), out.c_str());
+    Rprintf("%.*s", (int) buf.size(), out.c_str());
   } else {
     write_buf(buf, con);
   }