diff --git a/i3bar/src/xcb.c b/i3bar/src/xcb.c
index 06cff98d..ae447fbc 100644
--- a/i3bar/src/xcb.c
+++ b/i3bar/src/xcb.c
@@ -1816,6 +1816,8 @@ void reconfig_windows(bool redraw_bars) {
                                                                       bar_height);
 
             /* Set the WM_CLASS and WM_NAME (we don't need UTF-8) atoms */
+            char *class;
+            int len = sasprintf(&class, "%s%ci3bar%c", config.bar_id, 0, 0);
             xcb_void_cookie_t class_cookie;
             class_cookie = xcb_change_property(xcb_connection,
                                                XCB_PROP_MODE_REPLACE,
@@ -1823,8 +1825,8 @@ void reconfig_windows(bool redraw_bars) {
                                                XCB_ATOM_WM_CLASS,
                                                XCB_ATOM_STRING,
                                                8,
-                                               (strlen("i3bar") + 1) * 2,
-                                               "i3bar\0i3bar\0");
+                                               len,
+                                               class);
 
             char *name;
             sasprintf(&name, "i3bar for output %s", walk->name);