diff --git a/lib/govuk_schemas/random_content_generator.rb b/lib/govuk_schemas/random_content_generator.rb
index 621f179..ef318cb 100644
--- a/lib/govuk_schemas/random_content_generator.rb
+++ b/lib/govuk_schemas/random_content_generator.rb
@@ -18,7 +18,17 @@ def string_for_type(type)
       when "email"
         Faker::Internet.email
       else
-        raise "Unknown attribute type `#{type}`"
+        raise <<~DOC
+           Unsupported JSON schema type `#{type}`
+
+           Supported formats are:
+             - date-time
+             - uri
+             - email
+
+           This can be fixed by adding a type to the `string_for_type` method in
+          `lib/govuk_schemas/random_content_generator.rb` in https://github.com/alphagov/govuk_schemas
+        DOC
       end
     end
 
diff --git a/spec/lib/random_content_generator_spec.rb b/spec/lib/random_content_generator_spec.rb
index 1448fde..cde68ce 100644
--- a/spec/lib/random_content_generator_spec.rb
+++ b/spec/lib/random_content_generator_spec.rb
@@ -18,6 +18,12 @@
 
       expect(response).to eq(email)
     end
+
+    it "raises an error if the type is not present" do
+      expect {
+        GovukSchemas::RandomContentGenerator.new.string_for_type("duration")
+      }.to raise_error(/Unsupported JSON schema type `duration`/)
+    end
   end
 
   describe ".uri" do