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cv::Mat assignment to Scalar

I am getting unexpected behavior for assigning the elements of a matrix--it seems the last element of the matrix is not assigned properly. The following is a minimum code example and output.

typedef unsigned short ValueType;
cv::Mat matrix;
matrix.create(5, 1, CV_16U);
matrix = 6;

for (unsigned int idx = 0; idx < matrix.rows; idx++)
{
    std::cout << idx << ": " << matrix.at<ValueType>(idx, 1) << std::endl;
}

Output:

0: 6
1: 6
2: 6
3: 6
4: 0

I am running this on WIndows 7, OpenCV version 2.4.10 and 2.4.11 compiled with Visual Studio 2012, 64bit.

I can't imagine something like this would be a problem in OpenCV, so I'm assuming there's something else going on here. Does anyone know what it might be? It does not matter if I replace the assignment line with any of these or other variations or even using different element types in the matrix.

matrix = cv::Scalar(6);
matrix.setTo(6);