错误NU1002：项目XXX中的依赖项Npgsql 3.1.0-alpha5不支持框架.NETPlatform，Version = v5.4

Question

我有一个像这个`

的project.json

{
  "version": "1.0.0-*",
  "description": "XXX Class Library",
  "authors": [ "Class Library template" ],
  "tooling": {
    "defaultNamespace": "XXX"
  },   
  "dependencies": {
    "Npgsql": "3.1.0-*"
  }, 
  "frameworks": {
    "dotnet5.4": {
      "dependencies": {
        "Microsoft.CSharp": "4.0.1-beta-23516",
        "System.Collections": "4.0.11-beta-23516",
        "System.Linq": "4.0.1-beta-23516",
        "System.Runtime": "4.0.21-beta-23516",
        "System.Threading": "4.0.11-beta-23516",
        "System.Data.Common": "4.0.1-*"         
      }
    },
    "dnxcore50": {
      "dependencies": {
        "Microsoft.CSharp": "4.0.1-*",
        "System.Collections": "4.0.11-*",
        "System.Console": "4.0.0-*",
        "System.Linq": "4.0.1-*",
        "System.Threading": "4.0.11-*",
        "System.Data.Common": "4.0.1-*"
      }
    }
  }
}

当我运行dnu restore时，我没有错误，但是当我运行dnu build时出现此错误：

  

错误NU1002：项目XXX中的依赖关系Npgsql 3.1.0-alpha5   不支持框架.NETPlatform，Version = v5.4。

我该如何解决这个问题？

非常感谢