diff --git a/openmc/material.py b/openmc/material.py
index 2dbe691f525..86cc3d7939c 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -1418,6 +1418,9 @@ def get_activity(self, units: str = 'Bq/cm3', by_nuclide: bool = False,
         if volume is None:
             volume = self.volume
 
+        if units in {'Bq', 'Ci'} and volume is None:
+            raise ValueError(f"Volume must be set in order to compute activity in '{units}'.")
+
         if units == 'Bq':
             multiplier = volume
         elif units == 'Bq/cm3':
@@ -1474,6 +1477,8 @@ def get_decay_heat(self, units: str = 'W', by_nuclide: bool = False,
 
         if units == 'W':
             multiplier = volume if volume is not None else self.volume
+            if multiplier is None:
+                raise ValueError("Volume must be set in order to compute total decay heat.")
         elif units == 'W/cm3':
             multiplier = 1
         elif units == 'W/m3':
diff --git a/tests/unit_tests/test_material.py b/tests/unit_tests/test_material.py
index 911b8867f96..89dfc03ddd8 100644
--- a/tests/unit_tests/test_material.py
+++ b/tests/unit_tests/test_material.py
@@ -561,6 +561,10 @@ def test_get_activity():
     m1.add_element("Fe", 0.7)
     m1.add_element("Li", 0.3)
     m1.set_density('g/cm3', 1.5)
+    with pytest.raises(ValueError, match="Volume must be set"):
+        m1.get_activity(units='Bq')
+    with pytest.raises(ValueError, match="Volume must be set"):
+        m1.get_activity(units='Ci')
     # activity in Bq/cc and Bq/g should not require volume setting
     assert m1.get_activity(units='Bq/cm3') == 0
     assert m1.get_activity(units='Bq/g') == 0
@@ -619,6 +623,8 @@ def test_get_decay_heat():
     m1.add_nuclide("U235", 0.2)
     m1.add_nuclide("U238", 0.8)
     m1.set_density('g/cm3', 10.5)
+    with pytest.raises(ValueError, match="Volume must be set"):
+        m1.get_decay_heat(units='W')
     # decay heat in W/cc and W/g should not require volume setting
     assert m1.get_decay_heat(units='W/cm3') == 0
     assert m1.get_decay_heat(units='W/g') == 0