diff --git a/Inventor/include/CGAL/IO/Inventor_ostream.h b/Inventor/include/CGAL/IO/Inventor_ostream.h
index 609ff498517..d1657c15d22 100644
--- a/Inventor/include/CGAL/IO/Inventor_ostream.h
+++ b/Inventor/include/CGAL/IO/Inventor_ostream.h
@@ -59,7 +59,7 @@ public:
         // cerr or a file handle to /dev/null. The latter one would
         // mimick the behaviour that one can still use a stream with
         // an invalid stream, but without producing any output.
-        CGAL_assertion( m_os);
+        CGAL_assertion( m_os != 0 );
         return *m_os;
     }
 };
diff --git a/Inventor/include/CGAL/IO/VRML_2_ostream.h b/Inventor/include/CGAL/IO/VRML_2_ostream.h
index 20f1a2743a7..edc8ccb30de 100644
--- a/Inventor/include/CGAL/IO/VRML_2_ostream.h
+++ b/Inventor/include/CGAL/IO/VRML_2_ostream.h
@@ -53,7 +53,7 @@ public:
         // cerr or a file handle to /dev/null. The latter one would
         // mimick the behaviour that one can still use a stream with
         // an invalid stream, but without producing any output.
-        CGAL_assertion( m_os);
+        CGAL_assertion( m_os != 0 );
         return *m_os;
     }
 private: