running imdecode on a buffer containing jpeg image

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i am running imdecode on a buffer (string called str) containing a 4 channel jpeg. I do this:

    vector<uchar> vectordata(str.begin(), str.end());
    Mat mat = imdecode(vectordata, IMREAD_UNCHANGED);

the mat object has the correct size in terms of rows and cols for the image but it is only full of white pixels (255 for all r,g,b and a values. Any help would be great here :)