diff --git a/lib/strtoofft.c b/lib/strtoofft.c
index 83dea5c4b7..30deb8c05b 100644
--- a/lib/strtoofft.c
+++ b/lib/strtoofft.c
@@ -224,7 +224,7 @@ CURLofft curlx_strtoofft(const char *str, char **endp, int base,
 
   while(*str && ISBLANK(*str))
     str++;
-  if('-' == *str) {
+  if(('-' == *str) || (ISSPACE(*str))) {
     if(endp)
       *endp = (char *)str; /* didn't actually move */
     return CURL_OFFT_INVAL; /* nothing parsed */