OpenCV Error: Assertion failed (scn == 3 || scn == 4 line 7564

OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cv::cvtColor, file C:\builds\master_PackSlave-win64-vc12-shared\opencv\modules\imgproc\src\color.cpp, line 7564

cvtColor(src, dst, CV_BGR2GRAY);//, src为空，未赋值

可能全部变量声明后，又在mian函数中声明，调用函数中使用src时，调用的全局的，就为空值。