Index: trunk/psModules/src/imcombine/pmStack.c
===================================================================
--- trunk/psModules/src/imcombine/pmStack.c	(revision 35082)
+++ trunk/psModules/src/imcombine/pmStack.c	(revision 35165)
@@ -1418,5 +1418,7 @@
       }
       combined->image->data.F32[y][x] = stats->robustMedian;
-      combined->mask->data.PS_TYPE_IMAGE_MASK_DATA[y][x] = 0;
+      if (combined->mask) {
+	combined->mask->data.PS_TYPE_IMAGE_MASK_DATA[y][x] = 0;
+      }
 #if (0)
       if ((x == 59)&&(y > 40)&&(y < 50)) {