Unable to launch calibration window when doing monocular camera calibration

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I'm running kinetic on Ubuntu 16.04, and I attempted to follow the camera calibration tutorial, but I could not reliably get the calibration window to launch despite receiving no errors after I executed:

rosrun camera_calibration cameracalibrator.py --size 7x6 --square 0.015 image:=/camera/image_raw camera:=/usb_cam

But if I add --approximate value, it can run normally. I am confused because the option is used on stereo cameras.