https://golang.org/pkg/reflect/#Value says:

The zero Value represents no value. Its IsValid method returns false, its Kind method returns Invalid, its String method returns "", and all other methods panic. Most functions and methods never return an invalid value. If one does, its documentation states the conditions explicitly.

But,

FieldByName, FieldByNameFunc, MapIndex, MethodByName, Recv, SetMapIndex, TryRecv

only say "it returns the zero Value if it's fail".

For example:

MethodByName returns a function value corresponding to the method of v with the given name. The arguments to a Call on the returned function should not include a receiver; the returned function will always use v as the receiver. It returns the zero Value if no method was found.

"the zero Value" seems ambiguous to me.