diff --git a/lib/url.c b/lib/url.c
index eab1d6d55..35d2d046b 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -1039,8 +1039,9 @@ CURLcode Curl_setopt(struct SessionHandle *data, CURLoption option,
        *  Check that request length does not overflow the size_t type.
        */
 
-      if ((data->set.postfieldsize < 0) ||
-          (data->set.postfieldsize > (curl_off_t)((size_t)-1)))
+      if ((sizeof(curl_off_t) != sizeof(size_t)) &&
+          ((data->set.postfieldsize < 0) ||
+           (data->set.postfieldsize > (curl_off_t)((size_t)-1))))
         result = CURLE_OUT_OF_MEMORY;
       else {
         char * p;