Fread a ubyte file

I am trying to fread a file with the following command:

f = fopen('t10k-images-idx3-ubyte','r');
[a,count] = fread(f,4,'int32');

However, I am receiving Invalid file identifier. Use fopen to generate a valid file identifier. What exactly this mean?

[Walter Roberson]

What does

dir('t10k-images-idx3-ubyte*')

show?

Have you checked to be sure you are in the correct directory?