diff --git a/openmc/cell.py b/openmc/cell.py
index 36f772ff5..bb59b1d3a 100644
--- a/openmc/cell.py
+++ b/openmc/cell.py
@@ -503,7 +503,7 @@ class Cell(IDManagerMixin):
                     path = "./surface[@id='{}']".format(node.surface.id)
                     if memo and node.surface.id in memo['surfaces']:
                         return
-                    if memo:
+                    if memo is not None:
                         memo['surfaces'].add(node.surface.id)
                     xml_element.append(node.surface.to_xml_element())
 
diff --git a/openmc/lattice.py b/openmc/lattice.py
index 985fec86e..dff00306a 100644
--- a/openmc/lattice.py
+++ b/openmc/lattice.py
@@ -467,7 +467,7 @@ class Lattice(IDManagerMixin, metaclass=ABCMeta):
         if memo is None:
             memo = {}
 
-        # If no nemoize'd clone exists, instantiate one
+        # If no memoize'd clone exists, instantiate one
         if self not in memo:
             clone = deepcopy(self)
             clone.id = None
@@ -760,7 +760,7 @@ class RectLattice(Lattice):
         # If the element does contain the Lattice subelement, then return
         if memo and self._id in memo['lattices']:
             return
-        if memo:
+        if memo is not None:
             memo['lattices'].add(self._id)
 
         lattice_subelement = ET.Element("lattice")
@@ -1282,7 +1282,7 @@ class HexLattice(Lattice):
         # If the element does contain the Lattice subelement, then return
         if memo and self._id in memo['lattices']:
             return
-        if memo:
+        if memo is not None:
             memo['lattices'].add(self._id)
 
         lattice_subelement = ET.Element("hex_lattice")
diff --git a/openmc/universe.py b/openmc/universe.py
index 7a9411ecf..ea8c7f294 100644
--- a/openmc/universe.py
+++ b/openmc/universe.py
@@ -519,7 +519,7 @@ class Universe(IDManagerMixin):
             # If the cell was not already written, write it
             if memo and cell_id in memo['cells']:
                 continue
-            if memo:
+            if memo is not None:
                 memo['cells'].add(cell_id)
 
             # Create XML subelement for this Cell