Since it is possible that the internal error resulted from an unhandled user error, it is also a good idea to double-check the usage and parameterization of the block reporting this error."
The MathWorks MATLAB Release R12.1 is not compatible with System Generator 1.1
MathWorks made infrastructure changes between MathWorks R12 (MATLAB 6/Simulink 4) and MathWorks R12.1 (MATLAB 6.1/Simulink 4.1); unfortunately, these changes break the System Generator libraries and models. Consequently, System Generator users should not install MathWorks R12.1. The preferred version remains MathWorks R12 (MATLAB 6.0/Simulink 4.0), although System Generator 1.1 will also run with MATLAB R11.1/Simulink 3.0.
Xilinx System Generator v1.2 (released in November, 2001) is compatible with MathWorks R12.1.