diff --git a/mount/mount_unix.go b/mount/mount_unix.go
index 4053fbbe..964263fd 100644
--- a/mount/mount_unix.go
+++ b/mount/mount_unix.go
@@ -21,7 +21,7 @@ func Mount(device, target, mType, options string) error {
 
 // Unmount lazily unmounts a filesystem on supported platforms, otherwise does
 // a normal unmount. If target is not a mount point, no error is returned.
-func Unmount(target string) error {
+var Unmount = func(target string) error {
 	err := unix.Unmount(target, mntDetach)
 	if err == nil || err == unix.EINVAL { //nolint:errorlint // unix errors are bare
 		// Ignore "not mounted" error here. Note the same error
diff --git a/mount/mount_unix_test.go b/mount/mount_unix_test.go
index 1a703edd..3f345de4 100644
--- a/mount/mount_unix_test.go
+++ b/mount/mount_unix_test.go
@@ -4,6 +4,7 @@
 package mount
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path"
@@ -251,3 +252,42 @@ func TestRecursiveUnmountTooGreedy(t *testing.T) {
 		t.Fatal("expected dir-other to be mounted, but it's not")
 	}
 }
+
+func TestRecursiveUnmount_SubMountFailsToUnmount(t *testing.T) {
+	if os.Getuid() != 0 {
+		t.Skip("root required")
+	}
+
+	tmp, err := ioutil.TempDir("", t.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmp)
+
+	// Create a bunch of tmpfs mounts. Make sure "dir" itself is not
+	// a mount point, or we'll hit the fast path in RecursiveUnmount.
+	dirs := []string{"dir-other", "dir/subdir1", "dir/subdir1/subsub", "dir/subdir2/subsub"}
+	for _, d := range dirs {
+		dir := path.Join(tmp, d)
+		if err := os.MkdirAll(dir, 0o700); err != nil {
+			t.Fatal(err)
+		}
+		if err := Mount("tmpfs", dir, "tmpfs", ""); err != nil {
+			t.Fatal(err)
+		}
+		//nolint:errcheck
+		defer Unmount(dir)
+	}
+
+	var mockUnmount = func(target string) error {
+		return fmt.Errorf("error on calling unmount")
+	}
+	var originalUnmount = Unmount
+	Unmount = mockUnmount
+
+	// Unmount dir, make sure dir-other is still mounted.
+	if err := RecursiveUnmount(path.Join(tmp, "dir")); err == nil {
+		t.Fatal(err)
+	}
+	Unmount = originalUnmount
+}