Error: (-215:Assertion failed) !_filename.empty() in function 'cv::CvCapture_Images::open'

what does this error mean? i have googled this error but none of them got this error

error: (-215:Assertion failed) !_filename.empty() in function ‘cv::CvCapture_Images::open’

the error happens rarely. im using opencv python and pyqt

please show code, at least …