Writing an AST in VAX-Pascal

It has been many years since I wrote any VAX-Pascal and I find myself needing to write an AST. I have looked around in various documentation but can't find any examples. The AST I want to write is in response to the completion of a QIO. The QIO call looks like this: Stat := $QIO(0, Channel, IO$_WRITEVBLK, , %IMMED SendCompleted, 0, Buf, Len); And the AST itself looks like this (no body at the moment, just want to make it work first): [ASYNCHRONOUS] PROCEDURE SendCompleted( P : UNSIGNED); BEGIN END; When I run the code though I get a runtime error: %SYSTEM-F-ACCVIO, access violation, reason mask=00, virtual address=00000000, PC =000008C1, PSL=03C00000 %TRACE-F-TRACEBACK, symbolic stack dump follows module name routine name line rel PC abs PC DMC11SEND SENDCOMPLETED 67 00000009 000008C1 801D58F3 801D58F3 0001E893 0001E893 DMC11SEND SEND 77 00000070 00000938 DMC11SEND DMC11SEND 111 00000179 000006BD I am not sure what I have done wrong, anyone know? Thanks Rob