IWS "Low-order nibble of the byte at array offset 526 is not valid. Byte value: 00."

I can't figure out what is happening/wrong, nor do I know where the error message is coming from, nor where to look for the answer.

"Low-order nibble of the byte at array offset 526 is not valid. Byte value: 00."

I have a service program which reads a data record and loads the field values into a data structure defined like the record. That data structure is processed by an IWS wizard and the output is supplied as a web service.  When the web service is accessed from the web I get that error message.

I am suspicious that the issue revolves about issues with packed fields and zoned fields but can't prove it one way or another.