diff --git a/src/pg_rad/isotopes/isotope.py b/src/pg_rad/isotopes/isotope.py
index 9e7aa77..ec501af 100644
--- a/src/pg_rad/isotopes/isotope.py
+++ b/src/pg_rad/isotopes/isotope.py
@@ -40,7 +40,7 @@ class CS137(Isotope):
 
 
 preset_isotopes: Dict[str, Type[Isotope]] = {
-    "CS137": CS137
+    "Cs137": CS137
 }