diff --git a/src/azul/terraform.py b/src/azul/terraform.py
index 99c278f5e..13cf5c070 100644
--- a/src/azul/terraform.py
+++ b/src/azul/terraform.py
@@ -798,6 +798,8 @@ def tf_config(self, app_name):
         # specification lets us avoid maintaining a complicated trigger
         # dependency between Terraform resources.
         #
+        rest_api = resources['aws_api_gateway_rest_api'][app_name]
+        assert 'minimum_compression_size' not in rest_api, rest_api.keys()
         openapi_spec = json.loads(locals[app_name])
         key = 'x-amazon-apigateway-minimum-compression-size'
         openapi_spec[key] = config.minimum_compression_size