diff --git a/openapi2jsonschema/command.py b/openapi2jsonschema/command.py
index 9cd1bfc..42dc792 100644
--- a/openapi2jsonschema/command.py
+++ b/openapi2jsonschema/command.py
@@ -71,6 +71,8 @@ def default(output, schema, prefix, stand_alone, expanded, kubernetes, strict):
         version = data["swagger"]
     elif "openapi" in data:
         version = data["openapi"]
+    else:
+        raise ValueError("cannot convert data to JSON because we could not find 'openapi' or 'swagger' keys")
 
     if not os.path.exists(output):
         os.makedirs(output)