diff --git a/src/fr/inrialpes/exmo/align/onto/OntologyFactory.java b/src/fr/inrialpes/exmo/align/onto/OntologyFactory.java
index 50eec7e4cfc8e3c74b6c809328e46ec11f877d0f..ed2e4ca93d22fd1ed14070b991972d45c1ddf5af 100644
--- a/src/fr/inrialpes/exmo/align/onto/OntologyFactory.java
+++ b/src/fr/inrialpes/exmo/align/onto/OntologyFactory.java
@@ -75,8 +75,10 @@ public abstract class OntologyFactory {
     }
 
     public static void clear() {
-	for ( OntologyFactory of : instances.values() ){
-	    of.clearCache();
+	if ( instances != null ) {
+	    for ( OntologyFactory of : instances.values() ){
+		of.clearCache();
+	    }
 	}
     }