diff --git a/common/fileops.c b/common/fileops.c
index 0779a1f80..adcb1cf8a 100644
--- a/common/fileops.c
+++ b/common/fileops.c
@@ -161,7 +161,7 @@ sudo_parseln(FILE *fp)
 	len = strlen(buf);
 	while (len > 0 && isspace((unsigned char)buf[len - 1]))
 	    buf[--len] = '\0';
-	for (cp = buf; isblank(*cp); cp++)
+	for (cp = buf; isblank((unsigned char)*cp); cp++)
 	    continue;
     }
     return(cp);
diff --git a/plugins/sudoers/defaults.c b/plugins/sudoers/defaults.c
index d204da565..79feafcf3 100644
--- a/plugins/sudoers/defaults.c
+++ b/plugins/sudoers/defaults.c
@@ -669,13 +669,13 @@ store_list(char *str, struct sudo_defs_types *def, int op)
 	end = str;
 	do {
 	    /* Remove leading blanks, if nothing but blanks we are done. */
-	    for (start = end; isblank(*start); start++)
+	    for (start = end; isblank((unsigned char)*start); start++)
 		;
 	    if (*start == '\0')
 		break;
 
 	    /* Find end position and perform operation. */
-	    for (end = start; *end && !isblank(*end); end++)
+	    for (end = start; *end && !isblank((unsigned char)*end); end++)
 		;
 	    list_op(start, end - start, def, op == '-' ? delete : add);
 	} while (*end++ != '\0');