MATLAB进行灰度图像读取时： I=imread('lena.bmp'); IW=rgb2gray(I); 总报错

MATLAB进行灰度图像读取时： I=imread('lena.bmp'); IW=rgb2gray(I); 总报错? Error using ==> timesIntegers can only be combined with integers of the same class, or scalar doubles.Error in ==> rgb2gray at 65 I = X*coef;