It's not a bug. Interfaces are reference types. In this case, the interface is implemented by a pointer type. The interface can be thought of as a pointer to a nil pointer, but the "interface pointer" itself is not nil.

This is a tricky concept until you learn that interfaces are reference types.

Here's a simplified version of the previous example that illustrates this concept: https://play.golang.org/p/pODBgaHXq5