Index: trunk/ppStack/src/ppStackCombineFinal.c
===================================================================
--- trunk/ppStack/src/ppStackCombineFinal.c	(revision 23769)
+++ trunk/ppStack/src/ppStackCombineFinal.c	(revision 25078)
@@ -56,4 +56,7 @@
         }
         psKernel *covar = options->covariances->data[i]; // Covariance matrix
+        if (!covar) {
+            continue;
+        }
         float weight = options->weightings->data.F32[i]; // Weight to apply
         psBinaryOp(covar->image, covar->image, "*", psScalarAlloc(weight, PS_TYPE_F32));