diff --git a/src/cal/cal/sky2spectra.cal b/src/cal/cal/sky2spectra.cal
index 9354e4d3..3a5b1641 100644
--- a/src/cal/cal/sky2spectra.cal
+++ b/src/cal/cal/sky2spectra.cal
@@ -1,4 +1,4 @@
-{ RCSid $Id: sky2spectra.cal,v 1.3 2024/03/01 00:21:54 greg Exp $ }
+{ RCSid $Id: sky2spectra.cal,v 1.4 2024/05/08 18:43:12 greg Exp $ }
 {
 	Convert RGB sky to spectral sky using, for example:
 
@@ -32,4 +32,4 @@ in_M2 = M2(in_x,in_y);
 
 co_Multiplier = in_Y / (S0normf + in_M1*S1normf + in_M2*S2normf) / 179;
 
-co(n) = co_Multiplier * select(n, 1, in_M1, in_M2);
+co(n) = if(in_Y - 1e-4, co_Multiplier*select(n, 1, in_M1, in_M2), 0);