diff --git a/buildpacks/ruby/CHANGELOG.md b/buildpacks/ruby/CHANGELOG.md
index 16f49897..e7e2aebf 100644
--- a/buildpacks/ruby/CHANGELOG.md
+++ b/buildpacks/ruby/CHANGELOG.md
@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- The buildpack now warns the user when environmental variables used in running the default process are not defined. ([#307](https://github.com/heroku/buildpacks-ruby/pull/307))
+
 ## [3.0.0] - 2024-05-17
 
 ### Changed
diff --git a/buildpacks/ruby/src/steps/get_default_process.rs b/buildpacks/ruby/src/steps/get_default_process.rs
index 8683411c..b2dba89e 100644
--- a/buildpacks/ruby/src/steps/get_default_process.rs
+++ b/buildpacks/ruby/src/steps/get_default_process.rs
@@ -76,7 +76,7 @@ fn default_rack() -> Process {
             "-c",
             &[
                 "bundle exec rackup",
-                "--port \"$PORT\"",
+                "--port \"${PORT:?Error: PORT env var is not set!}\"",
                 "--host \"0.0.0.0\"",
             ]
             .join(" "),
@@ -91,7 +91,7 @@ fn default_rails() -> Process {
             "-c",
             &[
                 "bin/rails server",
-                "--port \"$PORT\"",
+                "--port \"${PORT:?Error: PORT env var is not set!}\"",
                 "--environment \"$RAILS_ENV\"",
             ]
             .join(" "),