I have the piece of code:

Medium_F.AbsolutePressure p_flow[numberOfFlueSections, numberOfTubeSections + 1](start = p_startFlow);

...

  for i in 1:numberOfFlueSections loop

    for j in 1:numberOfTubeSections loop

...

    C2[i, j] = deltaVFlow * drdp_flow[i, j] ;

...

    p_flow[i, j + 1] = p_flow[i, j] - Dpfric[i, j];

...

    stateFlow[i, j] = Medium_F.setState_ph(p_flow[i, j + 1], h_flow[i, j + 1]);

...

...

it is compiled and calculated normal.
But if I add to the code * der(p_flow[i, j + 1])

...

C2[i, j] = deltaVFlow * drdp_flow[i, j] * der(p_flow[i, j + 1]);

...

I have "Compilation process failed".

"C:\OpenModelica1.9.3\\MinGW\bin\mingw32-make.exe" -j4 -f MyHRSG.TestOnlyFlowHE.makefile
gcc    -falign-functions -msse2 -mfpmath=sse     -I"C:/OpenModelica1.9.3//include/omc/c" -I. -DOPENMODELICA_XML_FROM_FILE_AT_RUNTIME  -c -o MyHRSG.TestOnlyFlowHE.o MyHRSG.TestOnlyFlowHE.c
gcc    -falign-functions -msse2 -mfpmath=sse     -I"C:/OpenModelica1.9.3//include/omc/c" -I. -DOPENMODELICA_XML_FROM_FILE_AT_RUNTIME  -c -o MyHRSG.TestOnlyFlowHE_functions.o MyHRSG.TestOnlyFlowHE_functions.c
gcc    -falign-functions -msse2 -mfpmath=sse     -I"C:/OpenModelica1.9.3//include/omc/c" -I. -DOPENMODELICA_XML_FROM_FILE_AT_RUNTIME  -c -o MyHRSG.TestOnlyFlowHE_records.o MyHRSG.TestOnlyFlowHE_records.c
gcc    -falign-functions -msse2 -mfpmath=sse     -I"C:/OpenModelica1.9.3//include/omc/c" -I. -DOPENMODELICA_XML_FROM_FILE_AT_RUNTIME  -c -o MyHRSG.TestOnlyFlowHE_01exo.o MyHRSG.TestOnlyFlowHE_01exo.c
gcc    -falign-functions -msse2 -mfpmath=sse     -I"C:/OpenModelica1.9.3//include/omc/c" -I. -DOPENMODELICA_XML_FROM_FILE_AT_RUNTIME  -c -o MyHRSG.TestOnlyFlowHE_02nls.o MyHRSG.TestOnlyFlowHE_02nls.c
gcc    -falign-functions -msse2 -mfpmath=sse     -I"C:/OpenModelica1.9.3//include/omc/c" -I. -DOPENMODELICA_XML_FROM_FILE_AT_RUNTIME  -c -o MyHRSG.TestOnlyFlowHE_03lsy.o MyHRSG.TestOnlyFlowHE_03lsy.c
MyHRSG.TestOnlyFlowHE_03lsy.c: In function 'MyHRSG_TestOnlyFlowHE_initialLinearSystem':
MyHRSG.TestOnlyFlowHE_03lsy.c:167: error: 'MyHRSG_TestOnlyFlowHE_functionJacNLSJac7_column' undeclared (first use in this function)
MyHRSG.TestOnlyFlowHE_03lsy.c:167: error: (Each undeclared identifier is reported only once
MyHRSG.TestOnlyFlowHE_03lsy.c:167: error: for each function it appears in.)
MyHRSG.TestOnlyFlowHE_03lsy.c:168: error: 'MyHRSG_TestOnlyFlowHE_initialAnalyticJacobianNLSJac7' undeclared (first use in this function)
MyHRSG.TestOnlyFlowHE_03lsy.c:180: error: 'MyHRSG_TestOnlyFlowHE_functionJacNLSJac8_column' undeclared (first use in this function)
MyHRSG.TestOnlyFlowHE_03lsy.c:181: error: 'MyHRSG_TestOnlyFlowHE_initialAnalyticJacobianNLSJac8' undeclared (first use in this function)
mingw32-make: *** [MyHRSG.TestOnlyFlowHE_03lsy.o] Error 1
mingw32-make: *** Waiting for unfinished jobs....
Compilation process failed. Exited with code 2.

Why is this happening? Can I calculate the derivative without der()?