Generating a DLL shared library from a Simulink model with an S-function

I'm trying to generate a DLL from a Simulink fixed wing autopilot model which contains an S function written in Matlab. I follow the standard procedure using Embedded Coder but I end up with the following error:

For code generation, S-function parameters must be one or two dimensional nonsparse numeric or character arrays unless the S-function uses a TLC file and has an mdlRTW function.

Any leads to fix this issue would be great.

Thanks in advance.