diff --git a/src/net/http/fs.go b/src/net/http/fs.go
index 4f144ebad25..87caeb7e90f 100644
--- a/src/net/http/fs.go
+++ b/src/net/http/fs.go
@@ -818,6 +818,7 @@ func (f ioFile) Readdir(count int) ([]fs.FileInfo, error) {
 
 // FS converts fsys to a FileSystem implementation,
 // for use with FileServer and NewFileTransport.
+// The files provided by fsys must implement io.Seeker.
 func FS(fsys fs.FS) FileSystem {
 	return ioFS{fsys}
 }