how to fix not enough input arguments?

I created a faster rcnn called 'detector' and it detect people.I want to give the video feed from webcam and detect people in the video.this is my code.

cam = webcam();

videoFrame = snapshot(cam);

frameSize = size(videoFrame);

videoPlayer = vision.VideoPlayer;

runLoop = true;

frameCount = 0;

while runLoop && frameCount < 400

    videoFrame = snapshot(cam);
    frameCount = frameCount + 1;

    [bbox,scores] = detect(detector,step(videoFrame));
    if ~isempty(bbox)
        I = insertObjectAnnotation(videoFrame, 'rectangle', bbox, scores);
    end
    step(videoPlayer,I);
end
clear cam;
release(videoPlayer);

but I came up withis error.

Error using step

Not enough input arguments.

Error in webcamhumandetector (line 16)

    [bbox,scores] = detect(detector,step(videoFrame));

please help.