diff --git a/driver/mongodb/example/mongodb_test.go b/driver/mongodb/example/mongodb_test.go
index 33cecb9bb..c4371bf6d 100644
--- a/driver/mongodb/example/mongodb_test.go
+++ b/driver/mongodb/example/mongodb_test.go
@@ -232,7 +232,7 @@ func TestMigrate(t *testing.T) {
 				Organizations:    []Organization{},
 				Organizations_v2: []Organization_v2{},
 				Users:            []User{},
-				Errors:           []error{gomethods.MissingMethodError("v001_non_existing_method_up")},
+				Errors:           []error{gomethods.MethodNotFoundError("v001_non_existing_method_up")},
 			},
 		},
 		{
@@ -253,7 +253,7 @@ func TestMigrate(t *testing.T) {
 				Organizations:    []Organization{},
 				Organizations_v2: []Organization_v2{},
 				Users:            []User{},
-				Errors:           []error{gomethods.MethodNotExportedError("v001_not_exported_method_up")},
+				Errors:           []error{gomethods.MethodNotFoundError("v001_not_exported_method_up")},
 			},
 		},
 		{
diff --git a/driver/mongodb/gomethods/gomethods_migrator.go b/driver/mongodb/gomethods/gomethods_migrator.go
index 225f757d2..96fc1d50b 100644
--- a/driver/mongodb/gomethods/gomethods_migrator.go
+++ b/driver/mongodb/gomethods/gomethods_migrator.go
@@ -10,20 +10,16 @@ import (
 	"strings"
 )
 
-type MissingMethodError string
+type MethodNotFoundError string
 
-func (e MissingMethodError) Error() string { return "Non existing migrate method: " + string(e) }
+func (e MethodNotFoundError) Error() string {
+	return fmt.Sprintf("Method '%s' was not found. It is either not existing or has not been exported (starts with lowercase).", string(e))
+}
 
 type WrongMethodSignatureError string
 
 func (e WrongMethodSignatureError) Error() string {
-	return fmt.Sprintf("Method %s has wrong signature", string(e))
-}
-
-type MethodNotExportedError string
-
-func (e MethodNotExportedError) Error() string {
-	return fmt.Sprintf("Method %s is not exported", string(e))
+	return fmt.Sprintf("Method '%s' has wrong signature", string(e))
 }
 
 type MethodInvocationFailedError struct {
@@ -32,7 +28,7 @@ type MethodInvocationFailedError struct {
 }
 
 func (e *MethodInvocationFailedError) Error() string {
-	return fmt.Sprintf("Method %s returned an error: %v", e.MethodName, e.Error)
+	return fmt.Sprintf("Method '%s' returned an error: %v", e.MethodName, e.Err)
 }
 
 type MigrationMethodInvoker interface {
diff --git a/driver/mongodb/gomethods/gomethods_migrator_test.go b/driver/mongodb/gomethods/gomethods_migrator_test.go
index fe6865e0b..298f803a6 100644
--- a/driver/mongodb/gomethods/gomethods_migrator_test.go
+++ b/driver/mongodb/gomethods/gomethods_migrator_test.go
@@ -16,7 +16,7 @@ type FakeGoMethodsInvoker struct {
 
 func (invoker *FakeGoMethodsInvoker) Validate(methodName string) error {
 	if methodName == "V001_some_non_existing_method_up" {
-		return MissingMethodError(methodName)
+		return MethodNotFoundError(methodName)
 	}
 
 	return nil
@@ -93,7 +93,7 @@ func TestMigrate(t *testing.T) {
 					`),
 			},
 			expectedInvokedMethods: []string{},
-			expectedErrors:         []error{MissingMethodError("V001_some_non_existing_method_up")},
+			expectedErrors:         []error{MethodNotFoundError("V001_some_non_existing_method_up")},
 		},
 		{
 			name: "up migration: failing method stops execution",
diff --git a/driver/mongodb/mongodb.go b/driver/mongodb/mongodb.go
index d077e98bf..026e68626 100644
--- a/driver/mongodb/mongodb.go
+++ b/driver/mongodb/mongodb.go
@@ -59,7 +59,7 @@ func init() {
 }
 
 type DbMigration struct {
-	Id      bson.ObjectId `bson:"_id,omitempty"`
+	Id      bson.ObjectId `bson:"_id"`
 	Version uint64        `bson:"version"`
 }
 
@@ -79,6 +79,15 @@ func (driver *Driver) Initialize(url string) error {
 	}
 	session.SetMode(mgo.Monotonic, true)
 
+	c := session.DB(driver.methodsReceiver.DbName()).C(MIGRATE_C)
+	err = c.EnsureIndex(mgo.Index{
+		Key:    []string{"version"},
+		Unique: true,
+	})
+	if err != nil {
+		return err
+	}
+
 	driver.Session = session
 	driver.migrator = gomethods.Migrator{MethodInvoker: driver}
 
@@ -144,10 +153,10 @@ func (driver *Driver) Migrate(f file.File, pipe chan interface{}) {
 func (driver *Driver) Validate(methodName string) error {
 	methodWithReceiver, ok := reflect.TypeOf(driver.methodsReceiver).MethodByName(methodName)
 	if !ok {
-		return gomethods.MissingMethodError(methodName)
+		return gomethods.MethodNotFoundError(methodName)
 	}
 	if methodWithReceiver.PkgPath != "" {
-		return gomethods.MethodNotExportedError(methodName)
+		return gomethods.MethodNotFoundError(methodName)
 	}
 
 	methodFunc := reflect.ValueOf(driver.methodsReceiver).MethodByName(methodName)
@@ -164,7 +173,7 @@ func (driver *Driver) Invoke(methodName string) error {
 	name := methodName
 	migrateMethod := reflect.ValueOf(driver.methodsReceiver).MethodByName(name)
 	if !migrateMethod.IsValid() {
-		return gomethods.MissingMethodError(methodName)
+		return gomethods.MethodNotFoundError(methodName)
 	}
 
 	retValues := migrateMethod.Call([]reflect.Value{reflect.ValueOf(driver.Session)})