diff --git a/serverless/service/__init__.py b/serverless/service/__init__.py
index 18ee77f..e876d18 100644
--- a/serverless/service/__init__.py
+++ b/serverless/service/__init__.py
@@ -57,7 +57,11 @@ def __init__(
         self.service = Identifier(name)
         self.package = Package(["!./**/**", f"{self.service.snake}/**"])
         self.variablesResolutionMode = 20210326
-        self.custom = YamlOrderedDict(vars="${file(./variables.yml):${sls:stage}}", **(custom or {}))
+        self.custom = YamlOrderedDict(**(custom or {}))
+
+        if "vars" not in custom.keys():
+            custom["vars"] = "${file(./variables.yml):${sls:stage}}"
+
         self.config = config or Configuration()
         self.regions = regions