diff --git a/src/fileio.c b/src/fileio.c
index 18bbffb3789..9742cf1a258 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -1828,6 +1828,9 @@ DEFUN ("copy-file", Fcopy_file, Scopy_file, 2, 6,
 If NEWNAME is a directory name, copy FILE to a like-named file under
 NEWNAME.
 
+For NEWNAME to be recognized as a directory name, it should name a
+directory and end in a slash.
+
 This function always sets the file modes of the output file to match
 the input file.
 
@@ -2253,6 +2256,9 @@ If file has names other than FILE, it continues to have those names.
 If NEWNAME is a directory name, rename FILE to a like-named file under
 NEWNAME.
 
+For NEWNAME to be recognized as a directory name, it should name a
+directory and end in a slash.
+
 Signal a `file-already-exists' error if a file NEWNAME already exists
 unless optional third argument OK-IF-ALREADY-EXISTS is non-nil.
 An integer third arg means request confirmation if NEWNAME already exists.