VS2015 warning code CS1701 assuming assembly reference matches ' ' matches identity ' ', you may need to supply runtime policy

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ASP.NET

MVC

VS2015

I've been getting this warning:

"Warning CS1701 Assuming assembly reference 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' used by 'System.Data.SqlClient' matches identity 'System.Runtime, Version=4.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' of 'System.Runtime', you may need to supply runtime policy"

The weird thing is that I had no such issue yesterday, only when I opened my file again today.
I'm currently using .NET Core MVC on VS2015.

What I have tried:

Under the project.json dependencies, I've tried to add "System.Runtime": "4.0.0" but ended up getting another warning that "expected System.Runtime >= 4.1.0 but received 4.0.0 .

Edit: I closed my VS and re-opened it and the warning was gone! It'd be great if anyone could explain what the issue was though