It looks like ujson handles infinity different to the native json encoder:

import ujson, json, numpy

a = np.array([1,2,3,4,numpy.inf])
b = json.dumps({"test" : a.tolist()})
# outputs '{"test": [1,2,3,4,5, Infinity]}'
b= ujson.dumps({"test" : a.tolist()})
# raises OverflowError: Invalid Inf value when encoding double

Similarly, an Overflow error is raised when NaN is encountered. It seems the relevant lines are 499-508 in ultrajsonenc.c:

    if (value == HUGE_VAL || value == -HUGE_VAL)
    {
        SetError (obj, enc, "Invalid Inf value when encoding double");
        return FALSE;
    }
    if (! (value == value)) 
    {
        SetError (obj, enc, "Invalid Nan value when encoding double");
        return FALSE;
    }

In the interests of keeping ujson as a drop-in replacement for json, may I suggest this is changed so that it converts the NaN / Infinity to strings (like json) and doesn't raise an error? The decoder would likely need to be changed too...

Cheers
Dan