diff --git a/.github/workflows/R-CMD-check.yaml b/.github/workflows/R-CMD-check.yaml
index ee65ccb5..8ca6f6bf 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'}
+          # new CRAN checks re: format specification
+          - {os: windows-latest, r: 'devel'}
 
           - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
           - {os: ubuntu-latest,   r: 'release'}
diff --git a/src/unicode_fopen.h b/src/unicode_fopen.h
index 483bc7de..9fd8aeab 100644
--- a/src/unicode_fopen.h
+++ b/src/unicode_fopen.h
@@ -34,7 +34,7 @@ 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);
@@ -56,7 +56,7 @@ 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);