Index: source/grains.cpp
===================================================================
--- source/grains.cpp	(revision 12088)
+++ source/grains.cpp	(working copy)
@@ -3646,7 +3646,9 @@
 		for( int i=ipr3a; i < ipr3b; i++ )
 			yzero[i] = gv.bin[nd]->y0b06[i];
 	}
-	for( int i=ilo; i < ihi; i++ )
+	// result may underflow to zero if anu(ilo) is very close to threshold
+	ASSERT( yzero[ilo] >= 0.f );
+	for( int i=ilo+1; i < ihi; i++ )
 		ASSERT( yzero[i] > 0.f );
 }