Custom DLL - MissingMethodException: string[] string.Split

For some reason, I can't get string.split to work from within the dll. (I can successfully use it in Unity/C#)

Here is a snip of my C# code in a separate solution:

     using System;
     using System.Collections.Generic;
     
     namespace DLLPackage
     {
         public class TimeSheet : BaseObject
         {
             public TimeSheet(string builder)
             {
                 string[] elements = builder.Split(',');
             }
         }
 }

Everything compiles great. I put the dll in my assets/plugins folder.

 namespace UnityApp
 {
     public static class Portfolio
     {
         public static void Example()
         {
             TimeSheet t = new TimeSheet("-1,0,0,0");
 //MissingMethodException: string[] string.Split(char,System.StringSplitOptions)
 
 }
 }
 }

I have tried various cleans/rebuilds/renames that I found while googling. Nothing seems to help, it's ONLY this system method. I can call other system methods, my own methods, my own constructors no problem.