System.Runtime.Serialization.Json.DataContractJsonSerializer is not defined

I'm getting the error from the subject line when I try to move some code that formerly lived in the App_Code directory to an external library (that would be moved into the /bin directory).  What do I need to include or reference to get rid of this error?
I've already included:
System.Runtim.Serialization (says it's version 3.0.0, I don't see a version 3.5.0 anywhere). 
What am I missing?
TIA,
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