Help with error 'Cannot access index field ...'

I have a component that uses the command:
I have been running the component with no problem except that within
my application when accessing one form I get the error message:
'Cannot access index field semester'  if I pick a different menu item
that accesses the same component with all properties identical to the
first form that gives the error I don't get the error.

Does anyone know anything about this error message and what causes it?

Thanks, Rob