diff --git a/CGAL_Core/include/CGAL/CORE/BigFloatRep.h b/CGAL_Core/include/CGAL/CORE/BigFloatRep.h
index 25cfd243264..7439ce025a9 100644
--- a/CGAL_Core/include/CGAL/CORE/BigFloatRep.h
+++ b/CGAL_Core/include/CGAL/CORE/BigFloatRep.h
@@ -423,7 +423,7 @@ inline std::string BigFloatRep::toString(long prec, bool sci) const {
     else
       return r.rep;
   }
-  return nullptr;
+  return std::string();
 }
 
 inline void BigFloatRep::dump() const {