diff --git a/openmc/material.py b/openmc/material.py
index ea42d64b16..1341746dc0 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -86,6 +86,9 @@ class Material(IDManagerMixin):
     fissionable_mass : float
         Mass of fissionable nuclides in the material in [g]. Requires that the
         :attr:`volume` attribute is set.
+    activity : float
+        Activity of the material in [Bq]. Requires that the :attr:`volume`
+        attribute is set.
 
     """
 
@@ -152,7 +155,7 @@ class Material(IDManagerMixin):
         for key, value in atoms_per_barn_cm.items():
             half_life = openmc.data.half_life(key)
             if half_life:
-                total_activity += value[1] / half_life
+                total_activity += value / half_life
         total_activity *= math.log(2) * 1e24 * self.volume
 
         return total_activity