This error indicates that the key supplied to a Struct does not match the required regex.
Struct keys must satisfy the regex: [_a-zA-Z][_a-zA-Z0-9]*.
For example, the code below provides an erroneous key value:
const s = Struct([["$x ", UInt]]);
You can fix this by removing any illegal characters:
const s = Struct([["x", UInt]]);