diff --git a/i3bar/src/workspaces.c b/i3bar/src/workspaces.c
index 773f8f54..961a41f5 100644
--- a/i3bar/src/workspaces.c
+++ b/i3bar/src/workspaces.c
@@ -106,7 +106,7 @@ static int workspaces_string_cb(void *params_, const unsigned char *val, size_t
 
     if (!strcmp(params->cur_key, "name")) {
         const char *ws_name = (const char *)val;
-        params->workspaces_walk->canonical_name = strndup(ws_name, len);
+        params->workspaces_walk->canonical_name = sstrndup(ws_name, len);
 
         if (config.strip_ws_numbers && params->workspaces_walk->num >= 0) {
             /* Special case: strip off the workspace number */
diff --git a/include/libi3.h b/include/libi3.h
index a64b3981..3e6f8427 100644
--- a/include/libi3.h
+++ b/include/libi3.h
@@ -127,6 +127,13 @@ void *srealloc(void *ptr, size_t size);
  */
 char *sstrdup(const char *str);
 
+/**
+ * Safe-wrapper around strndup which exits if strndup returns NULL (meaning that
+ * there is no more memory available)
+ *
+ */
+char *sstrndup(const char *str, size_t size);
+
 /**
  * Safe-wrapper around asprintf which exits if it returns -1 (meaning that
  * there is no more memory available)
diff --git a/libi3/resolve_tilde.c b/libi3/resolve_tilde.c
index 3a56cbea..26cbabe5 100644
--- a/libi3/resolve_tilde.c
+++ b/libi3/resolve_tilde.c
@@ -23,7 +23,7 @@ char *resolve_tilde(const char *path) {
     char *head, *tail, *result;
 
     tail = strchr(path, '/');
-    head = strndup(path, tail ? (size_t)(tail - path) : strlen(path));
+    head = sstrndup(path, tail ? (size_t)(tail - path) : strlen(path));
 
     int res = glob(head, GLOB_TILDE, NULL, &globbuf);
     free(head);
diff --git a/libi3/safewrappers.c b/libi3/safewrappers.c
index 74460f37..f5973cab 100644
--- a/libi3/safewrappers.c
+++ b/libi3/safewrappers.c
@@ -48,6 +48,13 @@ char *sstrdup(const char *str) {
     return result;
 }
 
+char *sstrndup(const char *str, size_t size) {
+    char *result = strndup(str, size);
+    if (result == NULL)
+        err(EXIT_FAILURE, "strndup()");
+    return result;
+}
+
 int sasprintf(char **strp, const char *fmt, ...) {
     va_list args;
     int result;
diff --git a/src/window.c b/src/window.c
index 29cd11eb..7b0f397d 100644
--- a/src/window.c
+++ b/src/window.c
@@ -27,30 +27,27 @@ void window_update_class(i3Window *win, xcb_get_property_reply_t *prop, bool bef
      * null-terminated strings (for compatibility reasons). Instead, we
      * use strdup() on both strings */
     const size_t prop_length = xcb_get_property_value_length(prop);
-    char *new_class = smalloc(prop_length + 1);
-    memcpy(new_class, xcb_get_property_value(prop), prop_length);
-    new_class[prop_length] = '\0';
+    char *new_class = xcb_get_property_value(prop);
+    const size_t class_class_index = strnlen(new_class, prop_length) + 1;
 
     FREE(win->class_instance);
     FREE(win->class_class);
 
-    win->class_instance = sstrdup(new_class);
-    if ((strlen(new_class) + 1) < prop_length)
-        win->class_class = sstrdup(new_class + strlen(new_class) + 1);
+    win->class_instance = sstrndup(new_class, prop_length);
+    if (class_class_index < prop_length)
+        win->class_class = sstrndup(new_class + class_class_index, prop_length - class_class_index);
     else
         win->class_class = NULL;
     LOG("WM_CLASS changed to %s (instance), %s (class)\n",
         win->class_instance, win->class_class);
 
     if (before_mgmt) {
-        free(new_class);
         free(prop);
         return;
     }
 
     run_assignments(win);
 
-    free(new_class);
     free(prop);
 }