Index: trunk/psphot/src/by_SN.c
===================================================================
--- trunk/psphot/src/by_SN.c	(revision 4129)
+++ trunk/psphot/src/by_SN.c	(revision 4375)
@@ -9,4 +9,6 @@
     psF32 fA = (A->moments == NULL) ? 0 : A->moments->SN;
     psF32 fB = (B->moments == NULL) ? 0 : B->moments->SN;
+    if (isnan (fA)) fA = 0;
+    if (isnan (fB)) fB = 0;
 
     psF32 diff = fA - fB;