diff --git a/tools/ei-demo-client.c b/tools/ei-demo-client.c
index 16ce9a0..415ecc4 100644
--- a/tools/ei-demo-client.c
+++ b/tools/ei-demo-client.c
@@ -131,7 +131,8 @@ setup_xkb_keymap(struct ei_keymap *keymap)
 	for (unsigned int evcode = KEY_Q; evcode <= KEY_Y; evcode++) {
 		char utf8[7];
 		xkb_keysym_t keysym = xkb_state_key_get_one_sym(xkbstate, evcode + 8);
-		xkb_keysym_to_utf8(keysym, utf8, sizeof(utf8));
+		int len = xkb_keysym_to_utf8(keysym, utf8, sizeof(utf8));
+		assert(len > 0 && (size_t)len <= sizeof(utf8));
 		strcat(layout, utf8);
 	}