diff --git a/info.limpet/src/info/limpet/operations/spatial/TwoTrackOperation.java b/info.limpet/src/info/limpet/operations/spatial/TwoTrackOperation.java
index 56e3df06..5b1a0d8c 100644
--- a/info.limpet/src/info/limpet/operations/spatial/TwoTrackOperation.java
+++ b/info.limpet/src/info/limpet/operations/spatial/TwoTrackOperation.java
@@ -224,11 +224,18 @@ protected void recalculate(final IStoreItem subject)
     {
       // clear out the lists, first
       final DoubleDataset ds = performCalc();
-      final Document<?> output = getOutputs().get(0);
-      output.setDataset(ds);
+      if (!getOutputs().isEmpty())
+      {
+        final Document<?> output = getOutputs().get(0);
+        output.setDataset(ds);
 
-      // and fire updates
-      output.fireDataChanged();
+        // and fire updates
+        output.fireDataChanged();
+      }
+      else
+      {
+        System.err.println("no outputs");
+      }
     }
   }