diff --git a/lisp/textmodes/sgml-mode.el b/lisp/textmodes/sgml-mode.el
index 1672dce4f23..04f1dddf5fc 100644
--- a/lisp/textmodes/sgml-mode.el
+++ b/lisp/textmodes/sgml-mode.el
@@ -46,7 +46,8 @@
 
 (defcustom sgml-basic-offset 2
   "Specifies the basic indentation level for `sgml-indent-line'."
-  :type 'integer)
+  :type 'integer
+  :safe #'integerp)
 
 (defcustom sgml-attribute-offset 0
   "Specifies a delta for attribute indentation in `sgml-indent-line'.