Index: trunk/psModules/src/imcombine/pmSubtraction.c
===================================================================
--- trunk/psModules/src/imcombine/pmSubtraction.c	(revision 20141)
+++ trunk/psModules/src/imcombine/pmSubtraction.c	(revision 20215)
@@ -30,5 +30,5 @@
 #define MIN_SAMPLE_STATS    7           // Minimum number to use sample statistics; otherwise use quartiles
 
-#define SYS_ERROR 0.05                  // Relative error in derived convolution kernel pixels
+//#define SYS_ERROR 0.00                  // Relative error in derived convolution kernel pixels
 
 //////////////////////////////////////////////////////////////////////////////////////////////////////////////
@@ -462,4 +462,7 @@
                                        )
 {
+#ifndef SYS_ERROR
+    return NULL;
+#else
     int numCols = image->numCols, numRows = image->numRows; // Size of image
     psImage *sys = psImageAlloc(numCols, numRows, PS_TYPE_F32); // Systematic error image
@@ -472,4 +475,5 @@
 
     return sys;
+#endif
 }
 
@@ -1194,5 +1198,5 @@
     if (kernels->mode == PM_SUBTRACTION_MODE_1 || kernels->mode == PM_SUBTRACTION_MODE_DUAL) {
         sys1 = subtractionSysErrImage(ro1->image);
-        if (threaded) {
+        if (threaded && sys1) {
             psMutexInit(sys1);
         }
@@ -1200,5 +1204,5 @@
     if (kernels->mode == PM_SUBTRACTION_MODE_2 || kernels->mode == PM_SUBTRACTION_MODE_DUAL) {
         sys2 = subtractionSysErrImage(ro2->image);
-        if (threaded) {
+        if (threaded && sys2) {
             psMutexInit(sys2);
         }