Index: /trunk/ppStack/src/ppStackSources.c
===================================================================
--- /trunk/ppStack/src/ppStackSources.c	(revision 27446)
+++ /trunk/ppStack/src/ppStackSources.c	(revision 27447)
@@ -144,6 +144,5 @@
     for (int i = 0; i < num; i++) {
         psArray *sources = sourceLists->data[i]; // Source list
-        psAssert(sources, "Don't have source list %d", i);
-        if (sources->n == 0) {
+        if (!sources || 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;