Index: trunk/psModules/src/imcombine/pmSubtractionSimple.c
===================================================================
--- trunk/psModules/src/imcombine/pmSubtractionSimple.c	(revision 35774)
+++ trunk/psModules/src/imcombine/pmSubtractionSimple.c	(revision 35775)
@@ -355,7 +355,8 @@
     if (conv1->parent) {
       conv1->parent->data_exists = true;
-    }
-    if (conv1->parent->parent) {
-      conv1->parent->parent->data_exists = true;
+      
+      if (conv1->parent->parent) {
+	conv1->parent->parent->data_exists = true;
+      }
     }
   }
@@ -365,7 +366,7 @@
     if (conv2->parent) {
       conv2->parent->data_exists = true;
-    }
-    if (conv2->parent->parent) {
-      conv2->parent->parent->data_exists = true;
+      if (conv2->parent->parent) {
+	conv2->parent->parent->data_exists = true;
+      }
     }
   }