diff --git a/setup.py b/setup.py
index 27eb86b..5b12a20 100644
--- a/setup.py
+++ b/setup.py
@@ -27,6 +27,7 @@
 dependencies = [
     "google-api-core[grpc] >= 1.22.2, < 2.0.0dev",
     "proto-plus >= 1.4.0",
+    "packaging >= 14.3",
 ]
 
 package_root = os.path.abspath(os.path.dirname(__file__))
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 650f2c5..27906e5 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -21,3 +21,5 @@
 # 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