diff --git a/notfound/extension.py b/notfound/extension.py
index e7161ee..5c1228c 100644
--- a/notfound/extension.py
+++ b/notfound/extension.py
@@ -194,6 +194,13 @@ def process_doc(self, app, doctree):
         if sphinx.version_info >= (3, 0, 0):
             metadata.update({'nosearch': True})
 
+    def merge_other(self, app, env, docnames, other):
+        """Merge in specified data regarding docnames from a different `BuildEnvironment`
+        object which coming from a subprocess in parallel builds."""
+        # TODO: find an example about why this is strictly required for parallel read
+        # https://github.com/readthedocs/sphinx-notfound-page/pull/112/files#r498219556
+        env.metadata.update(other.metadata)
+            
 
 def handle_deprecated_configs(app, *args, **kwargs):
     """
diff --git a/tests/examples/parallel-build/conf.py b/tests/examples/parallel-build/conf.py
new file mode 100644
index 0000000..3485726
--- /dev/null
+++ b/tests/examples/parallel-build/conf.py
@@ -0,0 +1,11 @@
+"""
+Sample ``conf.py``.
+"""
+
+master_doc = 'index'
+
+extensions = [
+    'notfound.extension',
+]
+
+
diff --git a/tests/examples/parallel-build/five.rst b/tests/examples/parallel-build/five.rst
new file mode 100644
index 0000000..b03139a
--- /dev/null
+++ b/tests/examples/parallel-build/five.rst
@@ -0,0 +1,4 @@
+=======
+ Five
+=======
+
diff --git a/tests/examples/parallel-build/four.rst b/tests/examples/parallel-build/four.rst
new file mode 100644
index 0000000..e6ed1be
--- /dev/null
+++ b/tests/examples/parallel-build/four.rst
@@ -0,0 +1,4 @@
+=======
+ Four
+=======
+
diff --git a/tests/examples/parallel-build/index.rst b/tests/examples/parallel-build/index.rst
new file mode 100644
index 0000000..5c7cafc
--- /dev/null
+++ b/tests/examples/parallel-build/index.rst
@@ -0,0 +1,17 @@
+Samples for substitution directives
+===================================
+
+..
+
+  This is a test of parallel document builds. You need at least 5
+  documents. See:
+  https://github.com/adamtheturtle/sphinx-substitution-extensions/pull/173
+
+.. toctree::
+   :hidden:
+
+   one
+   two
+   three
+   four
+   five
diff --git a/tests/examples/parallel-build/one.rst b/tests/examples/parallel-build/one.rst
new file mode 100644
index 0000000..a5525d2
--- /dev/null
+++ b/tests/examples/parallel-build/one.rst
@@ -0,0 +1,4 @@
+=====
+ One
+=====
+
diff --git a/tests/examples/parallel-build/three.rst b/tests/examples/parallel-build/three.rst
new file mode 100644
index 0000000..1e4126d
--- /dev/null
+++ b/tests/examples/parallel-build/three.rst
@@ -0,0 +1,4 @@
+=======
+ Three
+=======
+
diff --git a/tests/examples/parallel-build/two.rst b/tests/examples/parallel-build/two.rst
new file mode 100644
index 0000000..ff1cabd
--- /dev/null
+++ b/tests/examples/parallel-build/two.rst
@@ -0,0 +1,4 @@
+=====
+ Two
+=====
+
diff --git a/tests/test_urls.py b/tests/test_urls.py
index 9f8e028..3e0d0f3 100644
--- a/tests/test_urls.py
+++ b/tests/test_urls.py
@@ -4,6 +4,7 @@
 import pytest
 import sphinx
 import shutil
+import subprocess
 import warnings
 
 srcdir = os.path.join(
@@ -27,6 +28,12 @@ def remove_sphinx_build_output():
             shutil.rmtree(build_path)
 
 
+@pytest.mark.sphinx(srcdir=srcdir)
+def test_parallel_build():
+    # TODO: migrate to `app.build(..., parallel=2)` after merging
+    # https://github.com/sphinx-doc/sphinx/pull/8257
+    subprocess.check_call('sphinx-build -j 2 -W -b html tests/examples/parallel-build build', shell=True)
+    
 @pytest.mark.sphinx(srcdir=srcdir)
 def test_404_page_created(app, status, warning):
     app.build()