RNF7535: varchar not compatible with options(*varsize)?

Compile of the following fails due to RNF7535 (The type and attributes of
parameter 1 do not match those of the prototype.)

I have stripped out a lot of the irrelevant code so that the compile
generates the same error. Variable DataToHash apparently doesn't match the
type and attributes of the first parameter, labeled "InputData", to the
hash procedure.

What am I missing?

**free
ctl-opt dftactgrp(*no);

dcl-s DataToHash varchar( 16773000);

dcl-pr hash extproc('Qc3CalculateHash');
InputData char( 16773000) options(*varsize);
LengthOfInputData int( 10);
end-pr;

dcl-s inputlen int( 10);

// stripped code that "builds" DataToHash

inputlen = %len(%trim( DataToHash));
hash( DataToHash: inputlen );
*inlr = *on;

- Dan