diff -up xdrawchem-1.9.9/xdrawchem/molecule_obmol.cpp.gcc6 xdrawchem-1.9.9/xdrawchem/molecule_obmol.cpp
--- xdrawchem-1.9.9/xdrawchem/molecule_obmol.cpp.gcc6	2005-11-30 03:55:06.000000000 +0100
+++ xdrawchem-1.9.9/xdrawchem/molecule_obmol.cpp	2016-02-17 16:03:30.652748203 +0100
@@ -198,7 +198,7 @@ OBMol * Molecule::convertToOBMol() {
     atom.SetType( type );
 
     if (!obmol->AddAtom(atom))
-      return(false);
+      return(NULL);
     atom.Clear();
   }
 
@@ -212,7 +212,7 @@ OBMol * Molecule::convertToOBMol() {
     bst = tmp_bond->Stereo();
     if (bst == STEREO_UP) flag |= OB_WEDGE_BOND;
     if (bst == STEREO_DOWN) flag |= OB_HASH_BOND;
-    if (!obmol->AddBond(start,end,order,flag)) return(false);
+    if (!obmol->AddBond(start,end,order,flag)) return(NULL);
   }
 
   obmol->EndModify();