diff --git a/usb/config.go b/usb/config.go
index 309a272..ffb3f5c 100644
--- a/usb/config.go
+++ b/usb/config.go
@@ -57,6 +57,9 @@ type Config struct {
 
 // Close releases the underlying device, allowing the caller to switch the device to a different configuration.
 func (c *Config) Close() error {
+	if c.dev == nil {
+		return nil
+	}
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	if len(c.claimed) > 0 {
@@ -98,6 +101,12 @@ func (c *Config) Interface(intf, alt int) (*Interface, error) {
 		return nil, fmt.Errorf("Inteface %d does not have alternate setting %d. Alt setting needs to be a 0-based index into the settings table, which has %d elements.", ifInfo, alt, len(ifInfo.AltSettings))
 	}
 
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.claimed[intf] {
+		return nil, fmt.Errorf("interface %d on %s is already claimed", intf, c)
+	}
+
 	// Claim the interface
 	if err := libusb.claim(c.dev.handle, uint8(intf)); err != nil {
 		return nil, fmt.Errorf("failed to claim interface %d on %s: %v", intf, c, err)
@@ -108,8 +117,6 @@ func (c *Config) Interface(intf, alt int) (*Interface, error) {
 		return nil, fmt.Errorf("failed to set alternate config %d on interface %d of %s: %v", alt, intf, c, err)
 	}
 
-	c.mu.Lock()
-	defer c.mu.Unlock()
 	c.claimed[intf] = true
 	return &Interface{
 		Setting: ifInfo.AltSettings[alt],
diff --git a/usb/device_test.go b/usb/device_test.go
index c55a700..20f47ea 100644
--- a/usb/device_test.go
+++ b/usb/device_test.go
@@ -35,27 +35,57 @@ func TestOpenEndpoint(t *testing.T) {
 	}
 	cfg, err := dev.Config(1)
 	if err != nil {
-		t.Fatalf("%s Config(1): %v", dev, err)
+		t.Fatalf("%s.Config(1): %v", dev, err)
 	}
 	defer cfg.Close()
 	intf, err := cfg.Interface(1, 1)
 	if err != nil {
-		t.Fatalf("%s Interface(1, 1): %v", cfg, err)
+		t.Fatalf("%s.Interface(1, 1): %v", cfg, err)
 	}
 	defer intf.Close()
 	got, err := intf.InEndpoint(6)
 	if err != nil {
-		t.Fatalf("InEndpoint(cfg=1, if=1, alt=1, ep=6IN): got error %v, want nil", err)
+		t.Fatalf("%s.InEndpoint(6): got error %v, want nil", intf, err)
 	}
 	if want := fakeDevices[1].Configs[0].Interfaces[1].AltSettings[1].Endpoints[1]; !reflect.DeepEqual(got.Info, want) {
-		t.Errorf("InEndpoint(cfg=1, if=1, alt=1, ep=6IN): got %+v, want %+v", got, want)
+		t.Errorf("%s.InEndpoint(6): got %+v, want %+v", intf, got, want)
+	}
+
+	if _, err := cfg.Interface(1, 0); err == nil {
+		t.Fatalf("%s.Interface(1, 0): got nil, want non nil, because Interface 1 is already claimed.", cfg)
+	}
+
+	// intf2 is interface #0, not claimed yet.
+	intf2, err := cfg.Interface(0, 0)
+	if err != nil {
+		t.Fatalf("%s.Interface(0, 0): got %v, want nil", cfg, err)
 	}
 
 	if err := cfg.Close(); err == nil {
-		t.Fatalf("cfg.Close(): returned nil, want non nil, because the Interface was not release.")
+		t.Fatalf("%s.Close(): got nil, want non nil, because the Interface was not released.", cfg)
 	}
 	if err := dev.Close(); err == nil {
-		t.Fatalf("dev.Close(): returned nil, want non nil, because the Config was not released.")
+		t.Fatalf("%s.Close(): got nil, want non nil, because the Config was not released.", cfg)
 	}
+
 	intf.Close()
+	if err := cfg.Close(); err == nil {
+		t.Fatalf("%s.Close(): got nil, want non nil, because the Interface was not released.", cfg)
+	}
+	if err := dev.Close(); err == nil {
+		t.Fatalf("%s.Close(): got nil, want non nil, because the Config was not released.", dev)
+	}
+
+	intf2.Close()
+	if err := dev.Close(); err == nil {
+		t.Fatalf("%s.Close(): got nil, want non nil, because the Config was not released.", dev)
+	}
+
+	if err := cfg.Close(); err != nil {
+		t.Fatalf("%s.Close(): got error %v, want nil", cfg, err)
+	}
+
+	if err := dev.Close(); err != nil {
+		t.Fatalf("%s.Close(): got error %v, want nil", dev, err)
+	}
 }