diff --git a/setup.py b/setup.py
index c84ed6ab..07ca1452 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 >= 1.4.0",
+        "packaging >= 14.3",
     ),
     python_requires=">=3.6",
     classifiers=[
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index a37a34af..05a62e3a 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==1.4.0
+packaging==14.3
+google-auth==1.24.0  # TODO: remove when google-auth>=1.25.0 is required through google-api-core