diff --git a/lisp/international/mule.el b/lisp/international/mule.el
index 3e6e6435719..3be1e9e2877 100644
--- a/lisp/international/mule.el
+++ b/lisp/international/mule.el
@@ -1514,6 +1514,7 @@ DECODING is the coding system to be used to decode input from the process,
 ENCODING is the coding system to be used to encode output to the process.
 
 For a list of possible coding systems, use \\[list-coding-systems]."
+  (declare (interactive-only set-process-coding-system))
   (interactive
    "zCoding-system for output from the process: \nzCoding-system for input to the process: ")
   (let ((proc (get-buffer-process (current-buffer))))