this method doesn't tell why it fails when coercion fails, here is a change proposed that will save many debugging hours. pushAsPointer: gen "by default, push argument as a bytearray ptr" "may accept: - nil - variable bytes oop - external address " | asm proxy notNil done oop notExternalAddress | asm := gen asm. proxy := gen proxy. done := asm uniqueLabelName: 'done'. oop := gen reserveTemp. "load argument oop" loader emitLoad: gen to: oop. "handle nils, if we care" gen optCoerceNilToNull ifTrue: [ notNil := asm uniqueLabelName: 'notNil'. proxy nilObject. asm cmp: asm EAX with: oop; jne: notNil; xor: asm EAX with: asm EAX; push: asm EAX; jmp: done; label: notNil. ]. "handle ExternalAddress, if we care" gen optAllowExternalAddressPtr ifTrue: [ | oopClass | oopClass := gen reserveTemp. notExternalAddress := asm uniqueLabelName: 'notExternalAddress'. proxy fetchClassOf: oop. asm mov: asm EAX to: oopClass. gen emitFetchClass: NBExternalAddress. asm cmp: asm EAX with: oopClass. asm jne: notExternalAddress. proxy fetchPointer: 0 ofObject: oop. asm push: asm EAX. asm jmp: done. asm label: notExternalAddress. gen releaseTemps: 1. ]. "the last case is a byte/word array, simply push a pointer to first indexable field of oop" gen optAllowByteArraysPtr ifTrue: [ proxy isBytesOrWords: oop ifNotJumpTo: gen failedLabel. proxy firstIndexableField: oop. asm push: asm EAX. asm jmp: done. ]. asm jmp: gen failedLabel. *<----- change to: gen failWithCode: NBPrimErrBadArgument .* asm label: done. gen releaseTemps: 1. -- Lic. Javier Pimás Ciudad de Buenos Aires