diff --git a/src/main/java/de/ids_mannheim/korap/query/serialize/CollectionQueryProcessor.java b/src/main/java/de/ids_mannheim/korap/query/serialize/CollectionQueryProcessor.java
index 1930a9a..dd8d9e0 100644
--- a/src/main/java/de/ids_mannheim/korap/query/serialize/CollectionQueryProcessor.java
+++ b/src/main/java/de/ids_mannheim/korap/query/serialize/CollectionQueryProcessor.java
@@ -66,8 +66,8 @@
             throw new NullPointerException("Parser has not been instantiated!");
         }
         log.info("Processing virtual collection query: " + query);
-        log.debug("ANTLR parse tree: " + tree.toStringTree(parser));
         if (tree != null) {
+            log.debug("ANTLR parse tree: " + tree.toStringTree(parser));
             processNode(tree);
         }
         else {
@@ -119,7 +119,7 @@
 
             if (!checkOperatorValueConformance(term)) {
                 addError(StatusCodes.INCOMPATIBLE_OPERATOR_AND_OPERAND, 
-                        "Operator "+match+" is not valid.");
+                        new String[]{"Operator "+match+" is not acceptable.", match});
                 requestMap = new HashMap<String, Object>();
                 return;
             }