The Compute() command was originally intended to turn formulas into their computed MathObject values, and so early versions did not return Formula objects. That was fixed some time ago (Back in the summer of 2007), so I suspect you have an older version of WeBWorK's pg library. You may want to update to a more recent version. (This change was in version 1.7 of pg/macros/Parser.pl)
Davide
WeBWorK Problems
"Compute" not behaving as expected
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