This line treats "null" and null as equivalent for default values, however that is not the case. Having a default value that is actually the string "null" will fail when creating a new Codec.

To reproduce simply try goavro.NewCodec(sample) where sample is :

{ "name": "test_str", "type": [ "string", "null" ], "default": "null" }

The above would pass if the union was defined with null type as its first element but that's a different case. This would fail as well if it was a simple string type.