diff --git a/i3-config-wizard/main.c b/i3-config-wizard/main.c
index 5d08ca21..ccd2ba9a 100644
--- a/i3-config-wizard/main.c
+++ b/i3-config-wizard/main.c
@@ -732,7 +732,7 @@ static void finish(void) {
 
         /* Skip leading whitespace */
         char *walk = line;
-        while (isspace(*walk) && walk < (line + len)) {
+        while (isspace((unsigned char)*walk) && walk < (line + len)) {
             /* Pre-output the skipped whitespaces to keep proper indentation */
             fputc(*walk, ks_config);
             walk++;
diff --git a/i3bar/src/child.c b/i3bar/src/child.c
index 740912f0..f6d7cf78 100644
--- a/i3bar/src/child.c
+++ b/i3bar/src/child.c
@@ -525,7 +525,7 @@ static void stdin_io_first_line_cb(int fd) {
 
 static bool isempty(char *s) {
     while (*s != '\0') {
-        if (!isspace(*s)) {
+        if (!isspace((unsigned char)*s)) {
             return false;
         }
         s++;
diff --git a/src/util.c b/src/util.c
index c0609987..228846fa 100644
--- a/src/util.c
+++ b/src/util.c
@@ -67,7 +67,7 @@ __attribute__((__const__)) bool rect_equals(const Rect a, const Rect b) {
 __attribute__((pure)) bool name_is_digits(const char *name) {
     /* positive integers and zero are interpreted as numbers */
     for (size_t i = 0; i < strlen(name); i++) {
-        if (!isdigit(name[i])) {
+        if (!isdigit((unsigned char)name[i])) {
             return false;
         }
     }