Question: Substitution with mod function doesn't work

Help!!

Why do the first two of the following 4 examples not work in Maple 15?

subs(m=21,`mod`(m, 4));
subs(m=21,m mod(4));
                               21
                               21
`mod`(21, 4);
21 mod(4);

                               1
                               1
Is there a (simple) workaround?

Thanks.

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