Index: trunk/ppStack/src/ppStackReadout.c
===================================================================
--- trunk/ppStack/src/ppStackReadout.c	(revision 13515)
+++ trunk/ppStack/src/ppStackReadout.c	(revision 13613)
@@ -80,4 +80,11 @@
         psArrayAdd(stack, ARRAY_BUFFER, data);
         psFree(data);                   // Drop reference
+
+        // Ensure there is a mask, or pmStackCombine will complain
+        if (!ro->mask) {
+            ro->mask = psImageAlloc(ro->image->numCols, ro->image->numRows, PS_TYPE_MASK);
+            psImageInit(ro->mask, 0);
+        }
+
         fileNum++;
     }