Index: trunk/ppStack/src/ppStackCombineFinal.c
===================================================================
--- trunk/ppStack/src/ppStackCombineFinal.c	(revision 28405)
+++ trunk/ppStack/src/ppStackCombineFinal.c	(revision 29552)
@@ -93,5 +93,13 @@
 
     // Sum covariance matrices
+    // the array may be defined, but no covariances actually supplied.
+    bool haveCovariances = false;
     if (covariances) {
+	for (int i = 0; i < covariances->n; i++) {
+	    haveCovariances |= (covariances->data[i] != NULL);
+	}
+    }
+
+    if (haveCovariances) {
         outRO->covariance = psImageCovarianceAverageWeighted(covariances, options->weightings);
     } else {