Code generation failed Undefined function 'eq' for input arguments of type 'struct'

When attempting to port a model from R2007b to R2011b, when generating code for the model, which used to produce code without a problem, I get this error message

Code generation failed Undefined function 'eq' for input arguments of type 'struct'

The biggest problem is, there is no reference to where in the model the problem occurs. The console shows progress like this

### Build procedure for model: 'SoC_Model_AGC_Harness' aborted due to an error.
### Starting build procedure for model: SoC_Model_AGC_Harness

Code Directory :
     "C:\WindRiver\workspace\Core\SoX_2011b\slprj\_sfprj\SoC_Model_AGC_Harness\lib_soc\sfun\src"

Machine (#82): "lib_soc"  Target : "sfun"

Chart "SOC_GetPackSOCPct" (#83):

.
     "c1_lib_soc.h"

       "c1_lib_soc.c"

Chart "SOC_GetMinCellSOCPct" (#92):

.
     "c2_lib_soc.h"

       "c2_lib_soc.c"

......

then ends with

Interface and Support files:

Code generation failed Undefined function 'eq' for input arguments of type 'struct'.

### Build procedure for model: 'SoC_Model_AGC_Harness' aborted due to an error.

One additional piece of information. This error condition seems to occur before the TLC process begins. I chose the option for debugging TLC execution, but the TLC debugger never gets entered before the error occurs and the build process stops.

Can you provide me with some information that I can use to track down the source of this error ?

[Babak]

Can you run the simulation? press F5. You need to figure out where in your model you used the function 'eq'.

[Rolfe Dlugy-Hegwer]

Is the following topic helpful: http://www.mathworks.co.kr/matlabcentral/answers/31198-error-undefined-function-eq