Having trouble adding a .NET CLR to SQL Server

using Microsoft.SqlServer.Server;
using System.Data.SqlTypes;
using System.Text.RegularExpressions;

namespace ASI.Utilities
{
	public class SqlRegex
	{
		[SqlFunction(Name = "Replace", IsDeterministic = true)]
		public static string Replace(SqlString input, SqlString pattern, SqlString replacement)
		{
			if (input.IsNull || pattern.IsNull || replacement.IsNull)
			{
				return null;
			}

			return Regex.Replace(input.ToString(), pattern.ToString(), replacement.ToString());
		}
	}
}

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sp_configure 'clr enabled', 1
RECONFIGURE WITH OVERRIDE
GO

CREATE SCHEMA [Regex] AUTHORIZATION [CON5USERROLE]
GO

IF EXISTS (SELECT 1 FROM sys.assemblies WHERE [name] = N'RegexFunctions') 
   DROP ASSEMBLY [RegexFunctions]
GO

CREATE ASSEMBLY [RegexFunctions] FROM 'C:\Temp\T-SQL Regex.dll' WITH PERMISSION_SET = SAFE
GO

CREATE FUNCTION dbo.RegexReplace
(
	@input VARCHAR(4000),
	@pattern VARCHAR(4000),
	@replacement VARCHAR(4000)
)
RETURNS VARCHAR(4000)
AS EXTERNAL NAME
	RegexFunctions.[ASI.Utilities.SqlRegex].Replace
GO

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CREATE FUNCTION for "RegexReplace" failed because T-SQL and CLR types for return value do not match.