diff --git a/libtiff/tif_webp.c b/libtiff/tif_webp.c
index 0753c00f..e7b0c15d 100644
--- a/libtiff/tif_webp.c
+++ b/libtiff/tif_webp.c
@@ -79,8 +79,8 @@ int TWebPDatasetWriter(const uint8_t* data, size_t data_size,
   
   if ( (tif->tif_rawcc + (tmsize_t)data_size) > tif->tif_rawdatasize ) {
     TIFFErrorExt(tif->tif_clientdata, module,
-                 "Buffer too small by %lu bytes.",
-                 tif->tif_rawcc + data_size - tif->tif_rawdatasize);
+                 "Buffer too small by " TIFF_SIZE_FORMAT " bytes.",
+                 (size_t) (tif->tif_rawcc + data_size - tif->tif_rawdatasize));
     return 0;
   } else {
     _TIFFmemcpy(tif->tif_rawcp, data, data_size);