Non-exhaustive patterns in basicTypes/Var.lhs

QuickCheck's 'generate' function works fine in GHCi, but only for the _first_ time I call it. After that, I get an error: | Ok, modules loaded: Main. | *Main> generate 3 (mkStdGen 28) (return 'x') | Loading package QuickCheck ... linking ... done. | 'x' | | *Main> generate 3 (mkStdGen 28) (return 'x') | *** Exception: basicTypes/Var.lhs:226:32-58: Non-exhaustive | patterns in record update Peter