diff --git a/src/main/org/openscience/cdk/smiles/SmilesGenerator.java b/src/main/org/openscience/cdk/smiles/SmilesGenerator.java
index 6fdc0ef3198..83bce90653a 100644
--- a/src/main/org/openscience/cdk/smiles/SmilesGenerator.java
+++ b/src/main/org/openscience/cdk/smiles/SmilesGenerator.java
@@ -1825,7 +1825,7 @@ private String generateMassString(IAtom a)
         }
 
         IIsotope majorIsotope = isotopeFactory.getMajorIsotope(a.getSymbol());
-		if (majorIsotope.getMassNumber() == a.getMassNumber())
+		if (majorIsotope == null || majorIsotope.getMassNumber() == a.getMassNumber())
 		{
 			return "";
 		} else if (a.getMassNumber() == null)