@@ -276,7 +276,7 @@ func MergeDaemonConfigurations(flagsConfig *Config, flags *pflag.FlagSet, config

 	}

 	if err := Validate(fileConfig); err != nil {

-		return nil, fmt.Errorf("file configuration validation failed (%v)", err)

+		return nil, fmt.Errorf("configuration validation from file failed (%v)", err)

 	}

 	// merge flags configuration on top of the file configuration

@@ -287,7 +287,7 @@ func MergeDaemonConfigurations(flagsConfig *Config, flags *pflag.FlagSet, config

 	// We need to validate again once both fileConfig and flagsConfig

 	// have been merged

 	if err := Validate(fileConfig); err != nil {

-		return nil, fmt.Errorf("file configuration validation failed (%v)", err)

+		return nil, fmt.Errorf("merged configuration validation from file and command line flags failed (%v)", err)

 	}

 	return fileConfig, nil

@@ -459,14 +459,12 @@ func Validate(config *Config) error {

 			return err

 		}

 	}

-

 	// validate MaxConcurrentDownloads

-	if config.IsValueSet("max-concurrent-downloads") && config.MaxConcurrentDownloads != nil && *config.MaxConcurrentDownloads < 0 {

+	if config.MaxConcurrentDownloads != nil && *config.MaxConcurrentDownloads < 0 {

 		return fmt.Errorf("invalid max concurrent downloads: %d", *config.MaxConcurrentDownloads)

 	}

-

 	// validate MaxConcurrentUploads

-	if config.IsValueSet("max-concurrent-uploads") && config.MaxConcurrentUploads != nil && *config.MaxConcurrentUploads < 0 {

+	if config.MaxConcurrentUploads != nil && *config.MaxConcurrentUploads < 0 {

 		return fmt.Errorf("invalid max concurrent uploads: %d", *config.MaxConcurrentUploads)

 	}

@@ -103,6 +103,38 @@ func TestDaemonConfigurationMergeConflicts(t *testing.T) {

 	}

 }

+func TestDaemonConfigurationMergeConcurrent(t *testing.T) {

+	f, err := ioutil.TempFile("", "docker-config-")

+	if err != nil {

+		t.Fatal(err)

+	}

+

+	configFile := f.Name()

+	f.Write([]byte(`{"max-concurrent-downloads": 1}`))

+	f.Close()

+

+	_, err = MergeDaemonConfigurations(&Config{}, nil, configFile)

+	if err != nil {

+		t.Fatal("expected error, got nil")

+	}

+}

+

+func TestDaemonConfigurationMergeConcurrentError(t *testing.T) {

+	f, err := ioutil.TempFile("", "docker-config-")

+	if err != nil {

+		t.Fatal(err)

+	}

+

+	configFile := f.Name()

+	f.Write([]byte(`{"max-concurrent-downloads": -1}`))

+	f.Close()

+

+	_, err = MergeDaemonConfigurations(&Config{}, nil, configFile)

+	if err == nil {

+		t.Fatalf("expected no error, got error %v", err)

+	}

+}

+

 func TestDaemonConfigurationMergeConflictsWithInnerStructs(t *testing.T) {

 	f, err := ioutil.TempFile("", "docker-config-")

 	if err != nil {

@@ -240,6 +272,7 @@ func TestValidateConfigurationErrors(t *testing.T) {

 }

 func TestValidateConfiguration(t *testing.T) {

+	minusNumber := 4

 	testCases := []struct {

 		config *Config

 	}{

@@ -264,6 +297,28 @@ func TestValidateConfiguration(t *testing.T) {

 				},

 			},

 		},

+		{

+			config: &Config{

+				CommonConfig: CommonConfig{

+					MaxConcurrentDownloads: &minusNumber,

+					// This is weird...

+					ValuesSet: map[string]interface{}{

+						"max-concurrent-downloads": -1,

+					},

+				},

+			},

+		},

+		{

+			config: &Config{

+				CommonConfig: CommonConfig{

+					MaxConcurrentUploads: &minusNumber,

+					// This is weird...

+					ValuesSet: map[string]interface{}{

+						"max-concurrent-uploads": -1,

+					},

+				},

+			},

+		},

 	}

 	for _, tc := range testCases {

 		err := Validate(tc.config)