Index: trunk/ppStack/src/ppStackThread.c
===================================================================
--- trunk/ppStack/src/ppStackThread.c	(revision 21366)
+++ trunk/ppStack/src/ppStackThread.c	(revision 21377)
@@ -83,5 +83,5 @@
             psError(PS_ERR_UNKNOWN, false, "Unable to open convolved files %s, %s, %s",
                     (char*)imageNames->data[i], (char*)maskNames->data[i], (char*)varianceNames->data[i]);
-            return false;
+            return NULL;
         }
     }