diff --git a/shaclgen/__main__.py b/shaclgen/__main__.py
index e292ad4..43222e7 100644
--- a/shaclgen/__main__.py
+++ b/shaclgen/__main__.py
@@ -13,6 +13,7 @@
 @click.option(
     "-o",
     "--ontology",
+    is_flag=True,
     help="input file(s) or URL(s) is a schema or ontology",
     default=False,
 )