CAN mask on mixed ID lengths

When using CAN with mixed standard and extended IDs, how do I set the mask register?
I wish to receive Standard ID messages from 0 to 15.

So I set the mask to 0x1FC3FFFF, which should mask all bits apart from bits 0-3 of the SID

However,  the mailbox is also accepting Extended ID messages with the correct bits in the SID field, but with ANY bits in the EID field as if it were not masked at all.

Please advise.