Short explanation: a nil pointer to a struct that implements an interfaces is a valid non-nil value of that interface. --- Long explanation: In Go, "error" i

Short explanation: a nil pointer to a struct that implements an interfaces is a valid non-nil value of that interface. --- Long explanation: In Go, "error" is an interface - i.e. a type that has an "Error() string" method defined on it. The custom defined struct, "DatabaseError" is not an interface - it's a struct, and it has an "Error() string" defined on it. Therefore, any value of "DatabaseError" (or "*DatabaseError") type fulfills the "error" interface, and can be cast to a non-nil "error". Even the nil pointer to "DatabaseError" - you can call methods on a nil pointer, therefore it's a valid non-nil interface. The problem in the code is the implicit cast of "*DatabaseError" struct into the "error" interface in line 21., which assumes that nil pointer is the same as nil interface. It isn't. The solution is either to 1) return "error" instead of "*DatabaseError" in "QueryDatabase" (https://go.dev/play/p/fb0e_4loDBf https://go.dev/play/p/fb0e_4loDBf), or 2) to explicitly check the return value of "QueryDatabase" for nil pointer before cast (https://go.dev/play/p/TgikAk1mSn0 https://go.dev/play/p/TgikAk1mSn0). I prefer the former approach, because even if you're free to write your own "error" interface implementation, you're still supposed to use it through the "error" interface, not the struct pointer directly. Interface is more than just a struct pointer. P.S. As I explained in the other comment, the intention of the original comment is to differentiate between kinds of errors through the type system. That's not the Go way - Go avoids type hierarchies as much as possible. The proper way to differentiate errors in Go is through values: https://go.dev/play/p/ddzhAqRgK_1 https://go.dev/play/p/ddzhAqRgK_1 Or, if the user insists on having a error type hierarchy, define a custom "Is(target error) bool" method on each custom error type: https://go.dev/play/p/gZmYgOq6wSo https://go.dev/play/p/gZmYgOq6wSo