why return notEOF error with error is nil?

[zhuliquan]

participle/nodes.go

Line 495 in 220c1db

return nil, &UnexpectedTokenError{Unexpected: *notEOF}

I am confused why return notEOF error with error is nil?

[alecthomas]

This is the negation node which matches anything but what is defined. In this case notEOF is a token that did not satisfy this requirement and is an error.