diff --git a/src/os/file.go b/src/os/file.go
index 541b432081..b8cf89ce76 100644
--- a/src/os/file.go
+++ b/src/os/file.go
@@ -404,6 +404,7 @@ var lstat = Lstat
 
 // Rename renames (moves) oldpath to newpath.
 // If newpath already exists and is not a directory, Rename replaces it.
+// If newpath already exists and is a directory, Rename returns an error.
 // OS-specific restrictions may apply when oldpath and newpath are in different directories.
 // Even within the same directory, on non-Unix platforms Rename is not an atomic operation.
 // If there is an error, it will be of type *LinkError.