ASPNET 5 - Web API/Class Library broken after RC1 update

I have a simple project structure for now.

1) Class Library Project

2) Web API project having project reference to Class Library

With Beta 8

Class Library Project Json looked as follows:

"frameworks": {
"dnx50": { },
"dnxcore50": {
"dependencies": {
"Microsoft.CSharp": "4.0.1-beta-23409",
"System.Collections": "4.0.11-beta-23409",
"System.Linq": "4.0.1-beta-23409",
"System.Runtime": "4.0.21-beta-23409",
"System.Threading": "4.0.11-beta-23409"
}
}
}

Web API Project Json looked as follows:

"frameworks": {
"dnx50": { },
"dnxcore50": { }
},

With above configuration my Web API calls on IISExpress works fine.

With RC 1 update

My Web API call on IISExpress are not working unless I reference framework 4.5.1/4.5.2 or 4.6 in class Library Project Json

If my Class Library project Json looks like below, Web API call doesn't work

"frameworks": {
"net50": { },
"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"
}
}
}

If I make my Class Library Project Json is as follows, Web API call works.

"frameworks": {
"net46": { },
"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"
}
}
}

Not sure why I have to reference Why this is the case? Why do we have to support 4.5.1/4.5.2 or 4.6 framework?