diff --git a/compose/environment-variables/set-environment-variables.md b/compose/environment-variables/set-environment-variables.md
index 2b81b3a554..0514f7a0cd 100644
--- a/compose/environment-variables/set-environment-variables.md
+++ b/compose/environment-variables/set-environment-variables.md
@@ -109,7 +109,11 @@ db:
 
 When you run `docker compose up` with this configuration, Compose looks for the `POSTGRES_VERSION` environment variable in the shell and substitutes its value in. For this example, Compose resolves the image to `postgres:9.3` before running the configuration.
 
-If an environment variable is not set, Compose substitutes with an empty string. In the example above, if `POSTGRES_VERSION` is not set, the value for the image option is `postgres:.`
+If an environment variable is not set, Compose substitutes with an empty string. In the example above, if `POSTGRES_VERSION` is not set, the value for the image option is `postgres:`.
+
+> **Note**
+>
+> `postgres:` is not a valid image reference. Docker expects either a reference without a tag, like `postgres` which defaults to the latest image, or with a tag such as `postgres:15`.
 
 > **Important**
 >