Include nil check (quii#287)

In the refactor we lose our nil check - should we not include this still?

maps.md

@@ -222,6 +222,10 @@ t.Run("unknown word", func(t *testing.T) {
 func assertError(t *testing.T, got, want error) {
     t.Helper()
 
+    if got == nil {
+        t.Fatal("expected to get an error.")
+    }
+
     if got != want {
         t.Errorf("got error %q want %q", got, want)
     }