some more defensive code for debugging low level issues on kiwi
diff --git a/libraries/kiwi/kiwi-sparql/src/main/java/org/apache/marmotta/kiwi/sparql/builder/SQLBuilder.java b/libraries/kiwi/kiwi-sparql/src/main/java/org/apache/marmotta/kiwi/sparql/builder/SQLBuilder.java
index 062ea1c..fb6d60a 100644
--- a/libraries/kiwi/kiwi-sparql/src/main/java/org/apache/marmotta/kiwi/sparql/builder/SQLBuilder.java
+++ b/libraries/kiwi/kiwi-sparql/src/main/java/org/apache/marmotta/kiwi/sparql/builder/SQLBuilder.java
@@ -702,7 +702,7 @@
             for(String v : bindings.getBindingNames()) {
                 SQLVariable sv = variables.get(v);
 
-                if(!sv.getExpressions().isEmpty()) {
+                if(sv != null && !sv.getExpressions().isEmpty()) {
                     List<String> vNames = sv.getExpressions();
                     String vName = vNames.get(0);
                     Value binding = converter.convert(bindings.getValue(v));
diff --git a/libraries/kiwi/kiwi-triplestore/src/main/java/org/apache/marmotta/kiwi/persistence/KiWiConnection.java b/libraries/kiwi/kiwi-triplestore/src/main/java/org/apache/marmotta/kiwi/persistence/KiWiConnection.java
index 650892d..128514d 100644
--- a/libraries/kiwi/kiwi-triplestore/src/main/java/org/apache/marmotta/kiwi/persistence/KiWiConnection.java
+++ b/libraries/kiwi/kiwi-triplestore/src/main/java/org/apache/marmotta/kiwi/persistence/KiWiConnection.java
@@ -1853,7 +1853,7 @@
             cacheNode(result);
             return result;
         } else {
-            throw new IllegalArgumentException("unknown node type in database result: "+ntype);
+            throw new IllegalArgumentException("unknown node type in database result for node id " + id + ": " + ntype);
         }
     }