diff --git a/lib/transfer.c b/lib/transfer.c
index d977013b1371cccf263b765eb3f385ab9332d118..929d6ab25d54b27d9bf689c30388be3ca2adcf81 100644
--- a/lib/transfer.c
+++ b/lib/transfer.c
@@ -573,7 +573,7 @@ CURLcode Curl_readwrite(struct connectdata *conn,
 
               if(end) {
                 /* skip all trailing space letters */
-                for(; isspace(*end) && (end > start); end--);
+                for(; isspace((int)*end) && (end > start); end--);
 
                 /* get length of the type */
                 len = end-start+1;