diff --git a/src/os/os_test.go b/src/os/os_test.go
index 765797f5fb..d2e8ed5d82 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -2687,7 +2687,7 @@ func TestOpenFileKeepsPermissions(t *testing.T) {
 }
 
 func TestDirFS(t *testing.T) {
-	if err := fstest.TestFS(DirFS("./signal"), "signal.go", "internal/pty/pty.go"); err != nil {
+	if err := fstest.TestFS(DirFS("./testdata/dirfs"), "a", "b", "dir/x"); err != nil {
 		t.Fatal(err)
 	}
 }
diff --git a/src/os/testdata/dirfs/a b/src/os/testdata/dirfs/a
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/src/os/testdata/dirfs/b b/src/os/testdata/dirfs/b
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/src/os/testdata/dirfs/dir/x b/src/os/testdata/dirfs/dir/x
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/src/testing/fstest/testfs.go b/src/testing/fstest/testfs.go
index 4da6f04eed..a7f8007333 100644
--- a/src/testing/fstest/testfs.go
+++ b/src/testing/fstest/testfs.go
@@ -24,6 +24,7 @@ import (
 // It also checks that the file system contains at least the expected files.
 // As a special case, if no expected files are listed, fsys must be empty.
 // Otherwise, fsys must only contain at least the listed files: it can also contain others.
+// The contents of fsys must not change concurrently with TestFS.
 //
 // If TestFS finds any misbehaviors, it returns an error reporting all of them.
 // The error text spans multiple lines, one per detected misbehavior.