OverflowError is raised when Python integer doesn't fit in C integer type due to platform limitations. Different platforms have different limits. But in PyLong_AsUnsignedLong() only the upper limit is platform-depended. Negative integers always are not accepted. PyLong_AsUnsignedLong() is used for values that can be only non-negative. I think that ValueError is more appropriate in this case than OverflowError.