Why does StringifyUtf8 in an expression throw an error? Stringify doesn't. Am I missing something, or is this just a "That's just the way it is" situation?

Code:
Use Windows.pkg

UChar[] ucaJson
Handle  hoJson
Boolean bOK

Get Create (RefClass(cJsonObject)) to hoJson
Get ParseString of hoJson '{"name": "Mike"}' to bOK
If bOK ;
    Get StringifyUtf8 of hoJson to ucaJson  // This works OK
If bOK ;
    Move (StringifyUtf8(hoJson)) to ucaJson // This throws an "Illegal Datatype Conversion" error
Mike