@Dennis, Why not...
    primitiveEnterCriticalSectionOnBehalfOf: activeProcess
         <primitive: 186>
         self primitiveFailed

I had been meaning to ask what this code at the top of the primitive was for...
    argumentCount > 0
        ifTrue:
            [criticalSection := self stackValue: 1.  "rcvr"
             activeProc := self stackTop]
        ifFalse:
            [criticalSection := self stackTop.  "rcvr"
             activeProc := self activeProcess].

but took a wild guess its to support onBehalfOf type calls.