Index: /trunk/psModules/src/imcombine/pmSubtractionMatch.c
===================================================================
--- /trunk/psModules/src/imcombine/pmSubtractionMatch.c	(revision 18200)
+++ /trunk/psModules/src/imcombine/pmSubtractionMatch.c	(revision 18201)
@@ -528,6 +528,6 @@
 
             // Set the variance factors
-            float xMid = 0.5 * (region->x0 + region->x1); // Average x position
-            float yMid = 0.5 * (region->y0 + region->y1); // Average y position
+            float xMid = 0.5 * (region ? (region->x0 + region->x1) : numCols); // Average x position
+            float yMid = 0.5 * (region ? (region->y0 + region->y1) : numRows); // Average y position
             switch (mode) {
               case PM_SUBTRACTION_MODE_1: {