diff --git a/src/include/stir/recon_buildblock/BackProjectorByBin.h b/src/include/stir/recon_buildblock/BackProjectorByBin.h
index ce63fb2e6d..6eb29b91df 100644
--- a/src/include/stir/recon_buildblock/BackProjectorByBin.h
+++ b/src/include/stir/recon_buildblock/BackProjectorByBin.h
@@ -169,7 +169,17 @@ void back_project(const RelatedViewgrams<float>&,
 
       If overriding this function in a derived class, you need to call this one.
    */
+  virtual void check(const ProjDataInfo& proj_data_info) const;
+
+  //! check if the arguments are the same as what was used for set_up()
+  /*! calls error() if anything is wrong.
+
+      If overriding this function in a derived class, you need to call this one.
+
+      Calls check(const ProjDataInfo&)
+   */
   virtual void check(const ProjDataInfo& proj_data_info, const DiscretisedDensity<3,float>& density_info) const;
+
   bool _already_set_up;
 
   //! Clone of the density sptr set with set_up()
diff --git a/src/include/stir/recon_buildblock/ForwardProjectorByBin.h b/src/include/stir/recon_buildblock/ForwardProjectorByBin.h
index dae51a1e51..c16c7ff881 100644
--- a/src/include/stir/recon_buildblock/ForwardProjectorByBin.h
+++ b/src/include/stir/recon_buildblock/ForwardProjectorByBin.h
@@ -141,7 +141,17 @@ virtual void set_up(
 
       If overriding this function in a derived class, you need to call this one.
    */
+  virtual void check(const ProjDataInfo& proj_data_info) const;
+
+  //! check if the arguments are the same as what was used for set_up()
+  /*! calls error() if anything is wrong.
+
+      If overriding this function in a derived class, you need to call this one.
+
+      Calls check(const ProjDataInfo&)
+   */
   virtual void check(const ProjDataInfo& proj_data_info, const DiscretisedDensity<3,float>& density_info) const;
+
   bool _already_set_up;
 
   //! The density ptr set with set_up()
diff --git a/src/recon_buildblock/BackProjectorByBin.cxx b/src/recon_buildblock/BackProjectorByBin.cxx
index d993e7afac..91e0c7d010 100644
--- a/src/recon_buildblock/BackProjectorByBin.cxx
+++ b/src/recon_buildblock/BackProjectorByBin.cxx
@@ -96,13 +96,20 @@ set_up(const shared_ptr<const ProjDataInfo>& proj_data_info_sptr,
 
 void
 BackProjectorByBin::
-check(const ProjDataInfo& proj_data_info, const DiscretisedDensity<3,float>& density_info) const
+check(const ProjDataInfo& proj_data_info) const
 {
   if (!this->_already_set_up)
     error("BackProjectorByBin method called without calling set_up first.");
   if (!(*this->_proj_data_info_sptr >= proj_data_info))
     error(boost::format("BackProjectorByBin set-up with different geometry for projection data.\nSet_up was with\n%1%\nCalled with\n%2%")
           % this->_proj_data_info_sptr->parameter_info() % proj_data_info.parameter_info());
+}
+
+void
+BackProjectorByBin::
+check(const ProjDataInfo& proj_data_info, const DiscretisedDensity<3,float>& density_info) const
+{
+  this->check(proj_data_info);
   if (! this->_density_sptr->has_same_characteristics(density_info))
     error("BackProjectorByBin set-up with different geometry for density or volume data.");
 }
@@ -265,7 +272,7 @@ back_project(const RelatedViewgrams<float>& viewgrams,
   if (!_density_sptr)
     error("You need to call start_accumulating_in_new_target() before back_project()");
 
-  check(*viewgrams.get_proj_data_info_sptr(), *_density_sptr);
+  check(*viewgrams.get_proj_data_info_sptr());
 
 #ifdef STIR_OPENMP
   const int thread_num=omp_get_thread_num();
diff --git a/src/recon_buildblock/ForwardProjectorByBin.cxx b/src/recon_buildblock/ForwardProjectorByBin.cxx
index db1eae7401..c9a436c3de 100644
--- a/src/recon_buildblock/ForwardProjectorByBin.cxx
+++ b/src/recon_buildblock/ForwardProjectorByBin.cxx
@@ -81,13 +81,20 @@ set_up(const shared_ptr<const ProjDataInfo>& proj_data_info_sptr,
 
 void
 ForwardProjectorByBin::
-check(const ProjDataInfo& proj_data_info, const DiscretisedDensity<3,float>& density_info) const
+check(const ProjDataInfo& proj_data_info) const
 {
   if (!this->_already_set_up)
     error("ForwardProjectorByBin method called without calling set_up first.");
   if (!(*this->_proj_data_info_sptr >= proj_data_info))
     error(boost::format("ForwardProjectorByBin set-up with different geometry for projection data.\nSet_up was with\n%1%\nCalled with\n%2%")
           % this->_proj_data_info_sptr->parameter_info() % proj_data_info.parameter_info());
+}
+
+void
+ForwardProjectorByBin::
+check(const ProjDataInfo& proj_data_info, const DiscretisedDensity<3,float>& density_info) const
+{
+  this->check(proj_data_info);
   if (! this->_density_sptr->has_same_characteristics(density_info))
     error("ForwardProjectorByBin set-up with different geometry for density or volume data.");
 }
@@ -254,7 +261,7 @@ forward_project(RelatedViewgrams<float>& viewgrams,
   if (!_density_sptr)
     error("You need to call set_input() forward_project()");
 
-  check(*viewgrams.get_proj_data_info_sptr(), *_density_sptr);
+  check(*viewgrams.get_proj_data_info_sptr());
 
   // first check symmetries
   {