After upgrading to webgrease 1.5.1.25624 from nuget, System.IO.FileLoadException is coming

I am asp.net death page, this is coming after i upgraded webgrease and bootstrap for mvc4 using nuget.

Server Error in '/' Application.

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Could not load file or assembly 'WebGrease' or one of its
dependencies. Strong name signature could not be verified. The
assembly may have been tampered with, or it was delay signed but not
fully signed with the correct private key. (Exception from HRESULT:
0x80131045) Description: An unhandled exception occurred during the
execution of the current web request. Please review the stack trace
for more information about the error and where it originated in the
code.

Exception Details: System.IO.FileLoadException: Could not load file
or assembly 'WebGrease' or one of its dependencies. Strong name
signature could not be verified. The assembly may have been tampered
with, or it was delay signed but not fully signed with the correct
private key. (Exception from HRESULT: 0x80131045)

Source Error:

An unhandled exception was generated during the execution of the
current web request. Information regarding the origin and location of
the exception can be identified using the exception stack trace below.

Assembly Load Trace: The following information can be helpful to
determine why the assembly 'WebGrease' could not be loaded.

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Version Information: Microsoft .NET Framework Version:4.0.30319;
ASP.NET Version:4.0.30319.18045