Index: trunk/psModules/src/imcombine/pmSubtraction.c
===================================================================
--- trunk/psModules/src/imcombine/pmSubtraction.c	(revision 19340)
+++ trunk/psModules/src/imcombine/pmSubtraction.c	(revision 19344)
@@ -1141,7 +1141,11 @@
                 psArrayAdd(args, 1, (pmReadout*)ro2); // Casting away const
                 // Since adding to the array can impact the reference count, we need to lock
-                psMutexLock(subMask);
+                if (subMask) {
+                    psMutexLock(subMask);
+                }
                 psArrayAdd(args, 1, (psImage*)subMask); // Casting away const
-                psMutexUnlock(subMask);
+                if (subMask) {
+                    psMutexUnlock(subMask);
+                }
                 PS_ARRAY_ADD_SCALAR(args, maskBad, PS_TYPE_U8);
                 PS_ARRAY_ADD_SCALAR(args, maskPoor, PS_TYPE_U8);