Perplexing compile error RNF7418 - embedded SQL

I have an SQL statement that is suppose to insert a record. When I compile the code I get a compile error that I can't quite make sense of:

<snip>
Code that creates the SQL Statement:

C/free
gSQLInsert = 'insert into punches employee, punchdate, '+
'punchtime, punch '+
'values(?,?,?,?)';
runSQL();
return SQLSTT;
/end-free
</snip>

RunSQL Code (with compile error)

<snip>
C/free
exec sql prepare s1 from :gSQLInsert;
exec sql
execute s1 using :employee, :punchdate, :punchtime, :punch;
RNF7418: The result field used in the EVAL operation cannot be modified.
return SQLSTT;
/end-free
</snip>

I am trying to figure out which field it is trying to modify. All the host variables here are input values for the insert parameters. I am not updating any fields, just passing in parameters. This is on IBM i 6.1. Member type is SQLRPGLE and I am using CRTSQLRPGI

What am I missing?