image pixel value multiplication with constant using customized multiplier function

I wrote a matlab code for a 16 bit multiplier, the function name is mul_16b_mari, checked its functionality and worked fine. I have to use this multiplier to perform image pixel multiplication with an constant 3. I tried by using the code which is given below but didn't work and got the error subscripted assignment dimension mismatch. Can someone help me to rectify this issue. The input image size is 512x512x3. The size of each pixel is 8 bit. I tried with an image of 16 bit pixel size too but got the same error.

clc;
clear all;
tic;
I = imread('image_output.bmp');
figure;
%imshow(I);
[m n]=size(I);
%I=im2double(I);
for i=1:m
    for j=1:n
        u=3;
        b(i,j)=mul_16b_mari(I(i,j),u)/65535;
        %b(i,j)=1-cos(b(i,j))/2;

         %mse=(1/(300*270))*(I(i,j)-b(i,j))^2;  

      end
  end

[KSSV]

Error could be in this function _mul_16b_mari, but it is not shown here??

[Adam]

Should be obvious if you use the debugger with a breakpoint on the line that your error message presumably points to, but which you didn't share with us.