Int64 coercing to Byte[] in stored procedure call

I've got a stored procedure that looks like this:

create procedure [dbo].[Batch_of_Things_Get] (@MaxTimestamp binary(8)) as 
begin
    set nocount on

    select top 3500
        i.ThingID,
        i.Name,
        i.CreationDate,
        i.local_timestamp
    from
        [dbo].[Thing] i
    where
        i.local_timestamp 开发者_JAVA百科> @MaxTimestamp
end

and some C# code (in .NET 3.5) that calls it via System.Data.SqlClient, using a long for the @MaxTimestamp parameter.

    public static string GetUpdatedThings(long MinTimestamp)
    {
        // the ExecuteXmlDocument call takes: string SPName, params object[] SPParams
        return new SqlProcedure(Config.ConnectionString).ExecuteXmlDocument("Batch_of_Things_Get", MinTimestamp).OuterXml;
    }

It used to work (and indeed is still working quite happily in production), however my dev copy is now throwing:

InvalidCastException: Failed to convert parameter value from a Int64 to a Byte[].

StackTrace:

at System.Data.SqlClient.SqlParameter.CoerceValue(Object value, MetaType destinationType)
at System.Data.SqlClient.SqlParameter.GetCoercedValue()
at System.Data.SqlClient.SqlParameter.Validate(Int32 index, Boolean isCommandProc)
at System.Data.SqlClient.SqlCommand.SetUpRPCParameters(_SqlRPC rpc, Int32 startCount, Boolean inSchema, SqlParameterCollection parameters)
at System.Data.SqlClient.SqlCommand.BuildRPC(Boolean inSchema, SqlParameterCollection  parameters, _SqlRPC& rpc)
at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async)
at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)
at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
at System.Data.SqlClient.SqlCommand.ExecuteXmlReader()

Now I can understand why this is failing now, but why not earlier?

Edit: I changed the C# code to convert the long to a Byte[], now it's throwing: Failed to convert parameter value from a Byte[] to a Int64., which is the opposite to what it was complaining about initially!

---

BitConverter.GetBytes is a handy utility function for this