diff --git a/openmc/mgxs/groups.py b/openmc/mgxs/groups.py
index db01e6125..3436c0e03 100644
--- a/openmc/mgxs/groups.py
+++ b/openmc/mgxs/groups.py
@@ -24,6 +24,8 @@ class EnergyGroups(object):
     ----------
     group_edges : Iterable of Real
         The energy group boundaries [MeV]
+    num_group : Integral
+        The number of energy groups
 
     """
 
@@ -233,4 +235,4 @@ class EnergyGroups(object):
         condensed_groups = EnergyGroups()
         condensed_groups.group_edges = group_edges
 
-        return condensed_groups
\ No newline at end of file
+        return condensed_groups