Index: trunk/ppStack/src/ppStackSources.c
===================================================================
--- trunk/ppStack/src/ppStackSources.c	(revision 27159)
+++ trunk/ppStack/src/ppStackSources.c	(revision 27162)
@@ -132,5 +132,7 @@
         psAssert(sources, "Don't have source list %d", i);
         if (sources->n == 0) {
+            psLogMsg("ppStack", PS_LOG_WARN, "Image %d has no sources for transparency measurement.", i);
             options->inputMask->data.PS_TYPE_VECTOR_MASK_DATA[i] |= PPSTACK_MASK_CAL;
+            zp->data.F32[i] = NAN;
             continue;
         }