Using eval on debug command line

Why do I get an "Assignment type error occurred."
when I code an eval on the debug command line that
attempts to copy a 5k byte field into a data structure?

Example:
Debug . . . EVAL chds = a5kfld

The identical statement executes in code, but not from
my debug command line.

I have a module that I use in rpgle where an sql statement has
parameter markers that must be replaced with a user-selected
value.

the fields passed to the module are 5k bytes and when I want
to view these fields in debug I have been coding an eval to copy
the long field to a DS that breaks the field into 1024 byte segments
I can display, but I would like to avoid the code, just include the
chds that breaks the field into 1024 byte segments, and enter
the EVAL from the command line.