从PHP调用COM方法时如何传递空指针？

There's this method definition in COM server IDL:

[id( 15 )] 
HRESULT RunProcessing(
    [in, defaultvalue( 0 )] IProcessingParams* ProcessingParams,
    [in, defaultvalue( 0 )] IIndicesCollection* Indices );

the COM server is implemented in C++ and proper registered and the typelib created from that IDL is also proper registered.

There's a piece of PHP code that tries to invoke that method and pass "null" as first parameter and a specific object as the second parameter:

$processor->RunProcessing( NULL, $indices );

This yields

Uncaught exception 'com_exception' with message 'Parameter 0: type mismatch'

and refers to the line with RunProcessing().

What's the proper way of having a null pointer passed into a COM object method from PHP code?