diff --git a/Dockerfile b/Dockerfile
index fe7fc7548..e8247182d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -6,7 +6,8 @@ ARG builder_image=ghcr.io/alphagov/govuk-ruby-builder:$ruby_version
 FROM $builder_image AS builder
 
 ENV DEVISE_PEPPER=unused \
-    DEVISE_SECRET_KEY=unused
+    DEVISE_SECRET_KEY=unused \
+    GOVUK_ENVIRONMENT_NAME=unused
 
 WORKDIR $APP_HOME
 COPY Gemfile* .ruby-version ./
diff --git a/app/models/govuk_environment.rb b/app/models/govuk_environment.rb
index 11ec64bf8..4f7298e38 100644
--- a/app/models/govuk_environment.rb
+++ b/app/models/govuk_environment.rb
@@ -3,7 +3,7 @@ def self.name
     if Rails.env.development? || Rails.env.test?
       "development"
     else
-      ENV["GOVUK_ENVIRONMENT_NAME"]
+      ENV.fetch("GOVUK_ENVIRONMENT_NAME")
     end
   end
 
diff --git a/config/initializers/govuk_admin_template.rb b/config/initializers/govuk_admin_template.rb
index 8bd10ed96..cdb9fc963 100644
--- a/config/initializers/govuk_admin_template.rb
+++ b/config/initializers/govuk_admin_template.rb
@@ -8,5 +8,5 @@
   c.disable_google_analytics = false
 end
 
-GovukAdminTemplate.environment_label = (GovukEnvironment.name || "development").titleize
+GovukAdminTemplate.environment_label = GovukEnvironment.name.titleize
 GovukAdminTemplate.environment_style = GovukEnvironment.production? ? "production" : "preview"
diff --git a/docs/environment-variables.md b/docs/environment-variables.md
index acb315e96..b9f0b8f1b 100644
--- a/docs/environment-variables.md
+++ b/docs/environment-variables.md
@@ -40,10 +40,6 @@ Used to configure `GovukAdminTemplate` and in `Healthcheck::ApiTokens#expiring_t
 
 * `GOVUK_ENVIRONMENT_NAME`
 
-Used in various bits of logic to detect the production instance of the app and used to display various environment-specific strings.
-
-* `INSTANCE_NAME`
-
 ## Splunk
 
 Used to stream event logs to Splunk for analysis of Signon usage patterns and anomalies.
diff --git a/test/models/govuk_environment_test.rb b/test/models/govuk_environment_test.rb
index 1dbbb9c79..50cec20fe 100644
--- a/test/models/govuk_environment_test.rb
+++ b/test/models/govuk_environment_test.rb
@@ -28,11 +28,18 @@ class GovukEnvironmentTest < ActionMailer::TestCase
       setup do
         Rails.env.stubs(:development?).returns(false)
         Rails.env.stubs(:test?).returns(false)
-        ENV.stubs(:[]).with("GOVUK_ENVIRONMENT_NAME").returns("govuk-environment-name")
       end
 
-      should "return value of GOVUK_ENVIRONMENT_NAME env var" do
-        assert_equal "govuk-environment-name", GovukEnvironment.name
+      should "return value of GOVUK_ENVIRONMENT_NAME if it is set" do
+        ClimateControl.modify(GOVUK_ENVIRONMENT_NAME: "govuk-environment-name") do
+          assert_equal "govuk-environment-name", GovukEnvironment.name
+        end
+      end
+
+      should "fail fast if GOVUK_ENVIRONMENT_NAME is not set" do
+        ClimateControl.modify(GOVUK_ENVIRONMENT_NAME: nil) do
+          assert_raises(KeyError) { GovukEnvironment.name }
+        end
       end
     end
   end