Index: trunk/ppStack/src/ppStackThread.c
===================================================================
--- trunk/ppStack/src/ppStackThread.c	(revision 21477)
+++ trunk/ppStack/src/ppStackThread.c	(revision 23190)
@@ -36,7 +36,13 @@
     psFree(stack->threads);
     for (int i = 0; i < stack->imageFits->n; i++) {
-        psFitsClose(stack->imageFits->data[i]);
-        psFitsClose(stack->maskFits->data[i]);
-        psFitsClose(stack->varianceFits->data[i]);
+        if (stack->imageFits->data[i]) {
+            psFitsClose(stack->imageFits->data[i]);
+        }
+        if (stack->maskFits->data[i]) {
+            psFitsClose(stack->maskFits->data[i]);
+        }
+        if (stack->varianceFits->data[i]) {
+            psFitsClose(stack->varianceFits->data[i]);
+        }
         stack->imageFits->data[i] = stack->maskFits->data[i] = stack->varianceFits->data[i] = NULL;
     }