Compile function with only some variables

I made the simple example below for explaining the problem. How can I compile only some variables?
For example, in the example only a and b are Integer, c can be any type.

In[218]:= cf = Compile[{{a, _Integer}, {b, _Integer}, c}, a*b + c]; cf[2, 3, d] CompiledFunction::cfsa: Argument d at position 3 should be a machine-size real number. Out[219]= 6 + d 

How can I solve this warning and make this work for any c?
I want "c" to be anything.