diff --git a/noxfile.py b/noxfile.py
index c5821ef..8366268 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -56,9 +56,7 @@ def lint(session):
     serious code quality issues.
     """
     session.install("flake8", BLACK_VERSION, "click<8.1.0")
-    session.run(
-        "black", "--check", *BLACK_PATHS,
-    )
+    session.run("black", "--check", *BLACK_PATHS)
     session.run("flake8", "google", "tests")
 
 
@@ -73,9 +71,7 @@ def blacken(session):
     check the state of the `gcp_ubuntu_config` we use for that Kokoro run.
     """
     session.install(BLACK_VERSION, "click<8.1.0")
-    session.run(
-        "black", *BLACK_PATHS,
-    )
+    session.run("black", *BLACK_PATHS)
 
 
 @nox.session(python=DEFAULT_PYTHON_VERSION)
@@ -189,7 +185,7 @@ def docs(session):
     """Build the docs for this library."""
 
     session.install("-e", ".")
-    session.install("sphinx", "alabaster", "recommonmark")
+    session.install("sphinx==4.0.1", "alabaster", "recommonmark")
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
     session.run(
@@ -211,7 +207,9 @@ def docfx(session):
     """Build the docfx yaml files for this library."""
 
     session.install("-e", ".")
-    session.install("sphinx", "alabaster", "recommonmark", "gcp-sphinx-docfx-yaml")
+    session.install(
+        "sphinx==4.0.1", "alabaster", "recommonmark", "gcp-sphinx-docfx-yaml"
+    )
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
     session.run(
diff --git a/setup.py b/setup.py
index 169a046..bc517c7 100644
--- a/setup.py
+++ b/setup.py
@@ -41,8 +41,9 @@
     include_package_data=True,
     install_requires=(
         "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
-        "libcst >= 0.2.5",
-        "proto-plus >= 1.4.0",
+        "libcst >= 0.2.5, <1.0.0dev",
+        "proto-plus >= 1.4.0, <2.0.0dev",
+        "protobuf<4.0.0dev",
     ),
     python_requires=">=3.6",
     classifiers=[