diff --git a/README.md b/README.md
index 87d2b96..5fa6c52 100644
--- a/README.md
+++ b/README.md
@@ -26,3 +26,5 @@ $ ./gen_core_extensions.py -n 5000
 ## main.py
 
 Does **NOT** have to be called from any specific directory!
+
+The `-i` (`--instance`) parameter has to be a core extension