OpenCV mean function crashes with mask created from OpenCV threshold function?

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I'm very new to OpenCV, so I guess I'm making some silly mistake.

First I'm creating an empty cv::mat matrix, mask, of CV_8U type and filling it with zeros. Then I'm filling the matrix with either 0's or 255's by checking the values in CV_32FC1 type cv::mat matrix croppedDifferenceImage by the use of cv::threshold. Then I'm using mask as a parameter for cv::mean function.

cv::Mat mask = cv::Mat(croppedDifferenceImage.rows, croppedDifferenceImage.cols, CV_8U, cv::Scalar(0));
cv::threshold(croppedDifferenceImage, mask, 3.2, 255, CV_THRESH_BINARY);
double mean = cv::mean(croppedDifferenceImage, mask)[0];

But I keep having a crash with the message:

/build/buildd/opencv-2.4.8+dfsg1/modules/core/src/stat.cpp:565: error: (-215) mask.empty() || mask.type() == CV_8U in function mean

What am I doing wrong?