need help for "no matching function for call to ..."

Question

code:

Observable<double,wf> Expectation_Value_X2(
      "obsX2", 1, Obs_Expectation_Value_X2, phii);
  for( int it = 0 ; it <= 2 ; it++ )
  {
    double time=dt*it;
    Expectation_Value_X2.measure(time);}

error:
Main.cpp:119: error: no matching function for call to 'Obs<double, wf, void>::measure(double&)'
measure.h:193: note: candidates are: R Obs<R, T, void>::measure() [with R = double, T = wf]

thanks a lot.