diff --git a/openmc/mgxs/mdgxs.py b/openmc/mgxs/mdgxs.py
index 2c616b3682..aab5d855be 100644
--- a/openmc/mgxs/mdgxs.py
+++ b/openmc/mgxs/mdgxs.py
@@ -1468,6 +1468,11 @@ class Beta(MDGXS):
        \beta_{d,g} &= \frac{\langle \nu^d \sigma_f \phi \rangle}
        {\langle \nu \sigma_f \phi \rangle}
 
+    NOTE: The Beta MGXS is the delayed neutron fraction computed directly from
+    the nuclear data. Often the delayed neutron fraction is
+    "importance-weighted" by the adjoint flux and called "beta-effective". It
+    is important to make clear that this Beta is not importance-weighted.
+
     Parameters
     ----------
     domain : openmc.Material or openmc.Cell or openmc.Universe or openmc.Mesh
diff --git a/openmc/mgxs_library.py b/openmc/mgxs_library.py
index d4acd94697..9718f8e2a5 100644
--- a/openmc/mgxs_library.py
+++ b/openmc/mgxs_library.py
@@ -1918,7 +1918,8 @@ class MGXSLibrary(object):
     @num_delayed_groups.setter
     def num_delayed_groups(self, num_delayed_groups):
         check_type('num_delayed_groups', num_delayed_groups, int)
-        check_greater_than('delayed_groups', delayed_groups, 0, equality=True)
+        check_greater_than('delayed_groups', num_delayed_groups, 0,
+                           equality=True)
         self._num_delayed_groups = num_delayed_groups
 
     def add_xsdata(self, xsdata):