diff --git a/setup.py b/setup.py
index 25d74c6d..03e8c9f6 100644
--- a/setup.py
+++ b/setup.py
@@ -47,6 +47,7 @@
     install_requires=(
         "google-api-core[grpc] >= 1.22.2, < 2.0.0dev",
         "proto-plus >= 1.10.0",
+        "packaging >= 14.3",
     ),
     python_requires=">=3.6",
     setup_requires=["libcst >= 0.2.5"],
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 69e1c139..23cdd2b8 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -7,3 +7,5 @@
 # Then this file should have foo==1.14.0
 google-api-core==1.22.2
 proto-plus==1.10.0
+packaging==14.3
+google-auth==1.24.0  # TODO: remove when google-auth>=1.25.0 is required through google-api-core