diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 8e5d1a2..d80b32e 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -3803,7 +3803,7 @@
 				/* end of string, end of loop */
 				*line = 0;
 				break;
-			} else if (isspace(*line)) {
+			} else if (isspace((unsigned char)*line)) {
 				newarg = 1;
 				*line = 0;
 			} else if (*line == '[') {