Index: trunk/ppStack/src/ppStackCombineFinal.c
===================================================================
--- trunk/ppStack/src/ppStackCombineFinal.c	(revision 25078)
+++ trunk/ppStack/src/ppStackCombineFinal.c	(revision 25096)
@@ -63,10 +63,12 @@
         sumWeights += weight;
     }
-    pmReadout *outRO = options->outRO;  // Output readout
-    outRO->covariance = psImageCovarianceSum(options->covariances);
-    psBinaryOp(outRO->covariance->image, outRO->covariance->image, "/",
-               psScalarAlloc(sumWeights, PS_TYPE_F32));
-    psFree(options->covariances); options->covariances = NULL;
-    psImageCovarianceTransfer(outRO->variance, outRO->covariance);
+    if (sumWeights > 0.0) {
+        pmReadout *outRO = options->outRO;  // Output readout
+        outRO->covariance = psImageCovarianceSum(options->covariances);
+        psBinaryOp(outRO->covariance->image, outRO->covariance->image, "/",
+                   psScalarAlloc(sumWeights, PS_TYPE_F32));
+        psFree(options->covariances); options->covariances = NULL;
+        psImageCovarianceTransfer(outRO->variance, outRO->covariance);
+    }
 
 #ifdef TESTING