initial stress subcase cannot be used as preloading

I need to apply initial stress on shell structure, and recover the deformed mesh to be assembled with other components using welds and bolts,

then do a normal modes analysis, everything i well and fine exept when I add the normal modes subcase and use the initial stress subcase as a preload I get the following error

*** ERROR # 6518 ***
 Subcase 1 takes initial stress and it cannot be used as a preloading
 subcase or an eigenvalue buckling referential subcase.

Theoretically, I cannot find anything in FEM that would prevent us from doing this, so is there a work around this

I tried to add second static subcase (dummy subcase) that continue the solution from the first subcase (initial stress subcase), then use the dummy subcase as a preload for the normal modes subcase but the same error msg apeared

Best Regards 

B.ZAOUAGUI