diff --git a/engine/examples/postgresql_service.md b/engine/examples/postgresql_service.md
index 6d3265bc7c..0ff7604200 100644
--- a/engine/examples/postgresql_service.md
+++ b/engine/examples/postgresql_service.md
@@ -21,7 +21,7 @@ suitably secure.
 # example Dockerfile for https://docs.docker.com/engine/examples/postgresql_service/
 #
 
-FROM ubuntu
+FROM ubuntu:16.04
 
 # Add the PostgreSQL PGP key to verify their Debian packages.
 # It should be the same key as https://www.postgresql.org/media/keys/ACCC4CF8.asc