diff --git a/aws_lambda/aws_lambda.py b/aws_lambda/aws_lambda.py
index cc69209..e37c76c 100755
--- a/aws_lambda/aws_lambda.py
+++ b/aws_lambda/aws_lambda.py
@@ -215,7 +215,7 @@ def invoke(
     env_vars = cfg.get('environment_variables')
     if env_vars:
         for key, value in env_vars.items():
-            os.environ[key] = get_environment_variable_value(value)
+            os.environ[key] = str(get_environment_variable_value(value))
 
     # Load and parse event file.
     path_to_event_file = os.path.join(src, event_file)