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

I have a component that uses the command:
        table.findkey([semester,hostkey]);
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
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email: RobDa...@wheel.demon.co
       Rob.Da...@unn.ac.uk
Web:   http://www.unn.ac.uk/~isu8