masking a proto_field of type uint64

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I have a field which has the following properties:

offset_in_bits: 6112
size_in_bits: 51

This means I need to i need to add the field in the following way:

pf_my_field = ProtoField.uint64(
    "my_protocol.my_field", "my_field",
    base.DEC,
    nil,
    0x7ffffffffffff)

The issue is that it doesn't work. the mask only works for the first 32 bits, but ignores the mask of the other 32. is this a bug? or am I missing something? note: I use masking for a lot of 32 bits or smaller fields and didn't have this kind of issue.