diff --git a/stack_orchestrator/util.py b/stack_orchestrator/util.py
index 98cfd611..97d48963 100644
--- a/stack_orchestrator/util.py
+++ b/stack_orchestrator/util.py
@@ -153,7 +153,7 @@ def get_parsed_deployment_spec(spec_file):
 def stack_is_external(stack: str):
     # Bit of a hack: if the supplied stack string represents
     # a path that exists then we assume it must be external
-    return Path(stack).exists()
+    return Path(stack).exists() if stack is not None else False
 
 
 def get_yaml():