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Your accumulator is empty because your frame size is empty. You should do something like this:

VideoCapture cap(0); // open the default camera
if (!cap.isOpened()) // check if we succeeded
    return -1;

// Variables
Mat frame;
Mat accumulator;
Mat scaled;    // Initialize the accumulator matrix
cap >> frame; // Now frame is not empty
accumulator = Mat::zeros(frame.size(), CV_32FC3);

while (1) {
    // Capture frame
    cap >> frame;

    // Get 50% of the new frame and add it to 50% of the accumulator
    accumulateWeighted(frame, accumulator, 0.5); //now no error here (i hope so)