npgsql 2.1.3和EF 6:无法确定存储版本;需要有效的存储连接或版本提示

我正在为PostgreSQL添加对EF 6的支持:PostgreSQL迁移生成器

我已经创建了一个测试项目来尝试它，但是当我创建一个新的连接return new NpgsqlConnection();时，它会向我抛出这个异常:

An exception of type 'System.NotSupportedException' occurred in EntityFramework.dll but was not handled in user code
Additional information: Unable to determine the provider name for provider factory of type 'Npgsql.NpgsqlFactory'. Make sure that the ADO.NET provider is installed or registered in the application config.

按照Npgsql官方网站的说明，我使用这个app.config文件:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
    <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
  </configSections>
  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
  </startup>
  <entityFramework>
    <providers>
      <provider invariantName="System.Data.SqlClient" type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer" />
      <provider invariantName="Npgsql" type="Npgsql.NpgsqlServices, Npgsql.EntityFramework"></provider>
    </providers>
    <defaultConnectionFactory type="Npgsql.NpgsqlConnectionFactory, Npgsql" />
  </entityFramework>
  <system.data>
    <DbProviderFactories>
      <remove invariant="Npgsql" />
      <add name="Npgsql Data Provider" invariant="Npgsql" support="FF" description=".Net Framework Data Provider for Postgresql" type="Npgsql.NpgsqlFactory, Npgsql" />
    </DbProviderFactories>
  </system.data>
</configuration>

我不明白为什么会触发这个异常。