SPlot crashes when yields are not the top level fitparameters

Hi,
When using the sPlot class it seems to be mandatory that the yields are RooRealVars and the fit Parameters. Using a Model like:
RooRealVar ratio("xsec ratio","",1,0,10);
RooRealVar yield_sm("yield_sm","",100);
RooFormulaVar Ns("Signal Yield","","@0*@1",RooArgSet(ratio,yield_sm));

causes a crash:
1 INFO:Minization -- RooMinuit::optimizeConst: deactivating const optimization
1) 0x8a18b88 RooFormulaVar:: nsig[ actualVars=(nsig_const,sig_ratio,lumi) formula="@0*@1*@2" ] = 160.748 "@0*@1*@2"
2) 0x8a19200 RooFormulaVar:: nbkg1[ actualVars=(nbkg1_const,bkg_scale,lumi) formula="@0*@1*@2" ] = 5275.91 "@0*@1*@2"
3) 0x8a19878 RooFormulaVar:: nbkg2[ actualVars=(nbkg2_const,bkg_scale,lumi) formula="@0*@1*@2" ] = 16498.5 "@0*@1*@2"
4) 0x8a19ef0 RooFormulaVar:: nbkg3[ actualVars=(nbkg3_const,bkg_scale,lumi) formula="@0*@1*@2" ] = 3774.23 "@0*@1*@2"
5) 0x8a1a568 RooFormulaVar:: nbkg4[ actualVars=(nbkg4_const,bkg_scale,lumi) formula="@0*@1*@2" ] = 8649.69 "@0*@1*@2"
1 INFO:NumericIntegration -- RooRealIntegral::init(hgg_pt_sig_landau_Int[ptVtx]) using numeric integrator RooIntegrator1D to calculate Int(ptVtx)
1 INFO:NumericIntegration -- RooRealIntegral::init(hgg_pt_bkg4_landau_Int[ptVtx]) using numeric integrator RooIntegrator1D to calculate Int(ptVtx)
EIGENVALUES OF SECOND-DERIVATIVE MATRIX:

MINUIT WARNING IN HESSE
============== MATRIX FORCED POS-DEF BY ADDING 0.001000 TO DIAGONAL.
MINUIT WARNING IN HESSE
============== Matrix inversion fails.
1 INFO:Minization -- RooFitResult::fillCorrMatrix: number of floating parameters is zero, correlation matrix not filled
1 INFO:Fitting -- RooAbsPdf::fitTo(model) Calculating sum-of-weights-squared correction matrix for covariance matrix
EIGENVALUES OF SECOND-DERIVATIVE MATRIX:

MINUIT WARNING IN HESSE
============== MATRIX FORCED POS-DEF BY ADDING 0.001000 TO DIAGONAL.
MINUIT WARNING IN HESSE
============== Matrix inversion fails.
1 INFO:Minization -- RooFitResult::fillCorrMatrix: number of floating parameters is zero, correlation matrix not filled

• • • Break *** segmentation violation
      Using host libthread_db library "/lib64/tls/libthread_db.so.1".
      Attaching to program: /proc/334/exe, process 334
      [Thread debugging using libthread_db enabled]
      [New Thread 4107560640 (LWP 334)]
      0xffffe410 in __kernel_vsyscall ()
      #1 0x00b7d4b3 in __waitpid_nocancel () from /lib/tls/libc.so.6
      #2 0x00b26779 in do_system () from /lib/tls/libc.so.6
      #3 0x00c5998d in system () from /lib/tls/libpthread.so.0
      #4 0xf7b623af in TUnixSystem::Exec ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #5 0xf7b68047 in TUnixSystem::StackTrace ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #6 0xf7b64a42 in TUnixSystem::DispatchSignals ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #7 0xf7b64ad0 in SigHandler ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #8 0xf7b63d4d in sighandler ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #9 <signal handler called>
      #10 0xf7aade4e in TObject::TObject$base ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #11 0xf6630e7e in TMatrixTSym<double>::TMatrixTSym ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libMatrix.so
      #12 0xf5ae2709 in RooAbsPdf::fitTo ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libRooFitCore.so
      #13 0xf5ada06b in RooAbsPdf::fitTo ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libRooFitCore.so
      #14 0xf55335d0 in RooStats::SPlot::AddSWeight ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libRooStats.so
      #15 0xf5535637 in RooStats::SPlot::SPlot ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libRooStats.so
      #16 0x08052920 in sPlot ()
      at /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/include/TObject.h:156
      #17 0x08052ed9 in main (argc=3, argv=0xffffca54) at makeyyPlots.cxx:116
    • glibc detected *** corrupted double-linked list: 0x08f974b8 *** *** Break *** segmentation violation
      Using host libthread_db library "/lib64/tls/libthread_db.so.1".
      Attaching to program: /proc/334/exe, process 334
      [Thread debugging using libthread_db enabled]
      [New Thread 4107560640 (LWP 334)]
      0xffffe410 in __kernel_vsyscall ()
      #1 0x00b7d4b3 in __waitpid_nocancel () from /lib/tls/libc.so.6
      #2 0x00b26779 in do_system () from /lib/tls/libc.so.6
      #3 0x00c5998d in system () from /lib/tls/libpthread.so.0
      #4 0xf7b623af in TUnixSystem::Exec ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #5 0xf7b68047 in TUnixSystem::StackTrace ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #6 0xf7b64a42 in TUnixSystem::DispatchSignals ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #7 0xf7b64ad0 in SigHandler ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #8 0xf7b63d4d in sighandler ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #9 <signal handler called>
      #10 0xf7aade4e in TObject::TObject$base ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libCore.so
      #11 0xf6630e7e in TMatrixTSym<double>::TMatrixTSym ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libMatrix.so
      #12 0xf5ae2709 in RooAbsPdf::fitTo ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libRooFitCore.so
      #13 0xf5ada06b in RooAbsPdf::fitTo ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libRooFitCore.so
      #14 0xf55335d0 in RooStats::SPlot::AddSWeight ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libRooStats.so
      #15 0xf5535637 in RooStats::SPlot::SPlot ()
      from /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/lib/libRooStats.so
      #16 0x08052920 in sPlot ()
      at /afs/cern.ch/user/r/ruthmann/scratch0/programs/root-dev/include/TObject.h:156
      #17 0x08052ed9 in main (argc=3, argv=0xffffca54) at makeyyPlots.cxx:116
    • glibc detected *** corrupted double-linked list: 0x08f974b8 ***

would be nice to have this working since ratios are more convenient to use.
regards, Nils