diff --git a/lib/url.c b/lib/url.c
index 5a11e423b..dec402c1d 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -1143,7 +1143,7 @@ CURLcode Curl_setopt(struct Curl_easy *data, CURLoption option,
      * figure it out. Enables binary posts.
      */
     bigsize = va_arg(param, long);
-    if(bigsize < 0)
+    if(bigsize < -1)
       return CURLE_BAD_FUNCTION_ARGUMENT;
 
     if(data->set.postfieldsize < bigsize &&
@@ -1162,6 +1162,8 @@ CURLcode Curl_setopt(struct Curl_easy *data, CURLoption option,
      * figure it out. Enables binary posts.
      */
     bigsize = va_arg(param, curl_off_t);
+    if(bigsize < -1)
+      return CURLE_BAD_FUNCTION_ARGUMENT;
 
     if(data->set.postfieldsize < bigsize &&
        data->set.postfields == data->set.str[STRING_COPYPOSTFIELDS]) {