diff --git a/src/window.c b/src/window.c
index ace109e3..5485bcc3 100644
--- a/src/window.c
+++ b/src/window.c
@@ -26,13 +26,16 @@ void window_update_class(i3Window *win, xcb_get_property_reply_t *prop, bool bef
     /* We cannot use asprintf here since this property contains two
      * null-terminated strings (for compatibility reasons). Instead, we
      * use strdup() on both strings */
-    char *new_class = xcb_get_property_value(prop);
+    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';
 
     FREE(win->class_instance);
     FREE(win->class_class);
 
     win->class_instance = sstrdup(new_class);
-    if ((strlen(new_class) + 1) < (size_t)xcb_get_property_value_length(prop))
+    if ((strlen(new_class) + 1) < prop_length)
         win->class_class = sstrdup(new_class + strlen(new_class) + 1);
     else
         win->class_class = NULL;
@@ -40,12 +43,14 @@ void window_update_class(i3Window *win, xcb_get_property_reply_t *prop, bool bef
         win->class_instance, win->class_class);
 
     if (before_mgmt) {
+        free(new_class);
         free(prop);
         return;
     }
 
     run_assignments(win);
 
+    free(new_class);
     free(prop);
 }
 
diff --git a/testcases/t/235-wm-class-change-handler.t b/testcases/t/235-wm-class-change-handler.t
index e6cacded..3685b30c 100644
--- a/testcases/t/235-wm-class-change-handler.t
+++ b/testcases/t/235-wm-class-change-handler.t
@@ -31,16 +31,17 @@ EOT
 my $pid = launch_with_config($config);
 
 sub change_window_class {
-    my ($window, $class) = @_;
+    my ($window, $class, $length) = @_;
     my $atomname = $x->atom(name => 'WM_CLASS');
     my $atomtype = $x->atom(name => 'STRING');
+    $length ||= length($class) + 1;
     $x->change_property(
         PROP_MODE_REPLACE,
         $window->id,
         $atomname->id,
         $atomtype->id,
         8,
-        length($class) + 1,
+        $length,
         $class
     );
     sync_with_i3;
@@ -65,6 +66,13 @@ is($con->{window_properties}->{instance}, 'special',
 is($con->{mark}, 'special_class_mark',
     'A `for_window` assignment should run for a match when the window changes class');
 
+change_window_class($win, "abcdefghijklmnopqrstuv\0abcd", 24);
+
+$con = @{get_ws_content($ws)}[0];
+
+is($con->{window_properties}->{class}, 'a',
+    'Non-null-terminated strings should be handled correctly');
+
 exit_gracefully($pid);
 
 done_testing;