Visual Explain Question

Hi,
I'm a newbie to the AS400 platform with much database, SQL, and DB2
experience on other platforms.  I am trying to cut and paste SQL from RPG
programs into Navigator in the Run SQL Scripts to do a Visual Explain.

On other platforms, explains allow you to replace host variables in the
code with "?".  When I try that here I get a message which I can not seem
to find in any manuals, online help, etc.  Does anyone know how to do this?

Here is a generasized version of the program SQL and the message I receive
when I try the Explain.

SQL:
select max(colx)
from schema.tab1
where cola = ?
and colb = ?
and colc = ? ;

Mystery Error messsage:
The number of parameter values set or registered does not match the number
of parameters.

Connie K.