diff --git a/src/main/java/com/complexible/stardog/plan/aggregates/Agg.java b/src/main/java/com/complexible/stardog/plan/aggregates/Agg.java
index e712296..d5ea1b5 100644
--- a/src/main/java/com/complexible/stardog/plan/aggregates/Agg.java
+++ b/src/main/java/com/complexible/stardog/plan/aggregates/Agg.java
@@ -117,8 +117,10 @@ protected ValueOrError _getValue() {
                 return ValueOrError.General.of(output_value);
             }
         } finally {
-            instance.close();
-            instance = null;
+            if(instance != null) {
+                instance.close();
+                instance = null;
+            }
         }
     }