diff --git a/lib/wibox/widget/textbox.lua b/lib/wibox/widget/textbox.lua
index fc0d0cfbe..90a37fe6c 100644
--- a/lib/wibox/widget/textbox.lua
+++ b/lib/wibox/widget/textbox.lua
@@ -69,8 +69,8 @@ function textbox:set_markup(text)
     end
 
     local attr, parsed = Pango.parse_markup(text, -1, 0)
-    -- In case of error, attr is false and parsed is an error message
-    if not attr then error(parsed) end
+    -- In case of error, attr is false and parsed is a GLib.Error instance.
+    if not attr then error(parsed.message or tostring(parsed)) end
 
     self._markup = text
     self._layout.text = parsed
diff --git a/luaa.c b/luaa.c
index 7629b51ca..42e5f95bd 100644
--- a/luaa.c
+++ b/luaa.c
@@ -323,6 +323,10 @@ luaA_panic(lua_State *L)
 static int
 luaA_dofunction_on_error(lua_State *L)
 {
+#if LUA_VERSION_NUM >= 502
+    /* Convert error to string, to prevent a follow-up error with lua_concat. */
+    luaL_tolstring(L, -1, NULL);
+#endif
     /* duplicate string error */
     lua_pushvalue(L, -1);
     /* emit error signal */