diff --git a/lisp/international/mule.el b/lisp/international/mule.el
index 8063ef68b7e..ddb2894daee 100644
--- a/lisp/international/mule.el
+++ b/lisp/international/mule.el
@@ -793,8 +793,9 @@ coding-system, to avoid infinite recursion.
 
 `:default-char'
 
-VALUE must be a character.  On encoding, a character not supported by
-the coding system is replaced with VALUE.
+VALUE must be a character.  On encoding, characters that are not
+supported by the coding system are each replaced with VALUE.  If
+not specified, the default is the space character #x20.
 
 `:for-unibyte'