There is no exception when accessing unimplemented registers in RV32E mode

There is a problem I've gotten very frequently when verifying a configuration in RV32E mode. Since this config has only 16 registers, it should rise an illegal instruction exception when it tries to access any register in the 16-31 range, but instead it executes the instruction normally with what it seems a random value. For example, mulh x5, x16, x5 doesn't rise an exception.