Using Linmod or Linearize across m-code in Simulink

I have a relatively simple m-code model that is embedded within a Simulink diagram. The m-code model could probably be represented by something about as simple as a low pass filter. When I try to use "linmod" or "linearize" across this m-code model embedded within Simulink, however, the model returned is nothing more than a gain. The state space model has an empty matrix for A, B, and C and there is just a gain for D. But it is clearly missing a pole location of the low pass filter. Is there any way to capture the pole, or any other states within the m-code within the returned model from linmod? Before writing, "why don't you just implement the low pass filter in Simulink?" it is for reasons that can't be avoided. We are receiving this m-code from another company that does not want us to touch "their model". So we are embedding this thing into our Simulink model, but "linmod" seems to be missing part of its dynamics.