letor_error

api_letor.cc: In function ‘bool test_ranker1()’:
api_letor.cc:126:47: error: no matching function for call to ‘Xapian::ListNETRanker::train_model(const char [18], const char [1])’
     ranker.train_model("training-data.txt", "");
                                               ^
In file included from ../include/xapian-letor.h:32:0,
                 from api_letor.cc:28:
../include/xapian-letor/ranker.h:260:10: note: candidate: virtual void Xapian::ListNETRanker::train_model(const std::vector<Xapian::FeatureVector>&)
     void train_model(const std::vector<Xapian::FeatureVector> & training_data);
          ^
../include/xapian-letor/ranker.h:260:10: note:   candidate expects 1 argument, 2 provided
Makefile:563: recipe for target 'api_letor.o' failed
make: *** [api_letor.o] Error 1