fix serialize exception.
fix serialize exception.

928PJY/docfx,sergey-vershinin/docfx,sergey-vershinin/docfx,sergey-vershinin/docfx,LordZoltan/docfx,LordZoltan/docfx,pascalberger/docfx,pascalberger/docfx,LordZoltan/docfx,superyyrrzz/docfx,DuncanmaMSFT/docfx,superyyrrzz/docfx,pascalberger/docfx,superyyrrzz/docfx,dotnet/docfx,DuncanmaMSFT/docfx,dotnet/docfx,dotnet/docfx,928PJY/docfx,hellosnow/docfx,928PJY/docfx,hellosnow/docfx,LordZoltan/docfx,hellosnow/docfx