Simulink Coder Error when compiling S-Function from S-Function Altera blocks

This message shows up every time I want to compile one or many S-Function blocks from Altera DSP builder into another S-Function:

Error: Simulink Coder Error in block: "untitled/Product1", block type "S-Function": User-defined data type Unknown is required for Root Level Input 0, but the information needed to define this data type in the generated S-Function code format is unknown.

Please help!