issue trying to run some examples

[cyrstem]

hey im getting this :
terminate called after throwing an instance of 'cv::Exception'
what(): OpenCV(4.1.0) /build/opencv/src/opencv-4.1.0/modules/objdetect/src/cascadedetect.cpp:1658: error: (-215:Assertion failed) !empty() in function 'detectMultiScale'
when trying to run a couple of examples