diff --git a/CHANGES.rst b/CHANGES.rst
index 6ff883b..e5ba6c1 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -3,6 +3,7 @@ Version 2.1.3
 
 Unreleased
 
+-   Allow Sphinx's parallel build feature. :issue:`88`
 
 Version 2.1.2
 -------------
diff --git a/src/pallets_sphinx_themes/themes/click/domain.py b/src/pallets_sphinx_themes/themes/click/domain.py
index 6b0ee40..a98709f 100644
--- a/src/pallets_sphinx_themes/themes/click/domain.py
+++ b/src/pallets_sphinx_themes/themes/click/domain.py
@@ -255,6 +255,11 @@ class ClickDomain(Domain):
     label = "Click"
     directives = {"example": DeclareExampleDirective, "run": RunExampleDirective}
 
+    def merge_domaindata(self, docnames, otherdata):
+        # Needed to support parallel build.
+        # Not using self.data -- nothing to merge.
+        pass
+
 
 def delete_example_runner_state(app, doctree):
     """Close and remove the :class:`ExampleRunner` instance once the
diff --git a/src/pallets_sphinx_themes/themes/jinja/domain.py b/src/pallets_sphinx_themes/themes/jinja/domain.py
index 9dabc1b..7723d18 100644
--- a/src/pallets_sphinx_themes/themes/jinja/domain.py
+++ b/src/pallets_sphinx_themes/themes/jinja/domain.py
@@ -258,6 +258,11 @@ class JinjaDomain(Domain):
         "nodes": NodesDirective,
     }
 
+    def merge_domaindata(self, docnames, otherdata):
+        # Needed to support parallel build.
+        # Not using self.data -- nothing to merge.
+        pass
+
 
 def setup(app):
     app.add_domain(JinjaDomain)