'Null regex not allowed Missing block

Got this simple grammar:

grammar H1 {
    token TOP { <h1> }
    token h1 { \#  }
}

Results in: Null regex not allowed Missing block

Google turns up next to nothing on it.

raku

Solution 1:^[1]

Still not sure why escaping doesn't work, but this does:

grammar H1 {
    token TOP { <h1> }
    token h1 { '#'  }
}

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Solution	Source
Solution 1	StevieD