diff --git a/plantuml-mode.el b/plantuml-mode.el
index e40ba0d..2cadfc4 100644
--- a/plantuml-mode.el
+++ b/plantuml-mode.el
@@ -144,7 +144,9 @@
   :group 'plantuml)
 
 (defcustom plantuml-indent-level 8
-  "Indentation level of PlantUML lines")
+  "Indentation level of PlantUML lines"
+  :type 'natnum
+  :group 'plantuml)
 
 (defun plantuml-jar-render-command (&rest arguments)
   "Create a command line to execute PlantUML with arguments (as ARGUMENTS)."