Index: trunk/psModules/src/imcombine/pmSubtractionSimple.c
===================================================================
--- trunk/psModules/src/imcombine/pmSubtractionSimple.c	(revision 35771)
+++ trunk/psModules/src/imcombine/pmSubtractionSimple.c	(revision 35774)
@@ -353,12 +353,20 @@
     conv1->analysis = psMetadataCopy(conv1->analysis, outAnalysis);
     conv1->data_exists = true;
-    conv1->parent->data_exists = true;
-    conv1->parent->parent->data_exists = true;
+    if (conv1->parent) {
+      conv1->parent->data_exists = true;
+    }
+    if (conv1->parent->parent) {
+      conv1->parent->parent->data_exists = true;
+    }
   }
   if (conv2) {
     conv2->analysis = psMetadataCopy(conv2->analysis, outAnalysis);
     conv2->data_exists = true;
-    conv2->parent->data_exists = true;
-    conv2->parent->parent->data_exists = true;
+    if (conv2->parent) {
+      conv2->parent->data_exists = true;
+    }
+    if (conv2->parent->parent) {
+      conv2->parent->parent->data_exists = true;
+    }
   }