expression mutator: switch case is not a constant expression

Checking fail: 
/home/gj/metamorphicCL/metamorphiccl/python/mmc_generate.py /home/gj/ocl/gcctestsuite/generic-c99/gcc.c-torture_execute_pr38051.c -o /home/gj/tmp/gcc.c-torture_execute_pr38051 --num-variants 2 --seed 805108 --percentageDead 15 --percentageExpr 10 --percentageGoto 20 --percentageSwitch 30 --noMultithreading --exitonerror 
#### 
/home/gj/metamorphicCL/metamorphiccl/python/mmc_check.py --dir /home/gj/tmp/gcc.c-torture_execute_pr38051 --cc gcc

error:

/home/gj/tmp/gcc.c-torture_execute_pr38051/variant_0.c:53:10: error: expression is not an integer
      constant expression
    case __IDENTITY_ROOT(
         ^~~~~~~~~~~~~~~~

Expression mutator is mutating case numbers generating compilation errors.