diff --git a/lib/roken/strptime.c b/lib/roken/strptime.c
index 1af4cb84a..8916fce5e 100644
--- a/lib/roken/strptime.c
+++ b/lib/roken/strptime.c
@@ -213,8 +213,8 @@ strptime (const char *buf, const char *format, struct tm *timeptr)
 	char *s;
 	int ret;
 
-	if (isspace (c)) {
-	    while (isspace (*buf))
+	if (isspace ((unsigned char)c)) {
+	    while (isspace ((unsigned char)*buf))
 		++buf;
 	} else if (c == '%' && format[1] != '\0') {
 	    c = *++format;