'argument' : ambiguous conversions from 'Foo *const ' to 'IUnknown *'

I've got an ATL class:

class Foo :
    public CComObjectRootEx<CComMultiThreadModel>,
    public CComCoClass<Foo, &CLSID_Foo>,
    public IPlugin,
    public IEventSubscriber
{
    // ...
};

I need to pass it to another object, like this:

pOther->MethodTakingIUnknown(this);

When I do this, I get the following error message:

error C2594: 'argument' : ambiguous conversions from 'Foo *const' to 'IUnknown *'

What am I doing wrong?