1 files changed, 6 insertions, 2 deletions

diff --git a/src/fns.c b/src/fns.c
index 8db2a86a725..2276a9971b2 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -2494,8 +2494,12 @@ mapcar1 (EMACS_INT leni, Lisp_Object *vals, Lisp_Object fn, Lisp_Object seq)
 DEFUN ("mapconcat", Fmapconcat, Smapconcat, 3, 3, 0,
        doc: /* Apply FUNCTION to each element of SEQUENCE, and concat the results as strings.
 In between each pair of results, stick in SEPARATOR.  Thus, " " as
-SEPARATOR results in spaces between the values returned by FUNCTION.
-SEQUENCE may be a list, a vector, a bool-vector, or a string.  */)
+  SEPARATOR results in spaces between the values returned by FUNCTION.
+SEQUENCE may be a list, a vector, a bool-vector, or a string.
+SEPARATOR must be a string.
+FUNCTION must be a function of one argument, and must return a value
+  that is a sequence of characters: either a string, or a vector or
+  list of numbers that are valid character codepoints.  */)
   (Lisp_Object function, Lisp_Object sequence, Lisp_Object separator)
 {
   USE_SAFE_ALLOCA;