The type checker is specified in Prolog and rejects the above scenario:

    instructionIsTypeSafe(fadd, Environment, _Offset, NextStackFrame, ExceptionStackFrame) :- 
        validTypeTransition(Environment, [float, float], float, StackFrame, NextStackFrame),
        exceptionStackFrame(StackFrame, ExceptionStackFrame).

Fun fact: Said type system has a 'top' type that is both the top type of the type system as well as the top half of a long or double, as those two actually take two values while everything else, including references, is only one value. Made some sense when everything was 32 bit, less so today.