Stored Procedure OUT parameter always returning NULL

My stored procedure returns the expected value, but the OUT parameter is not returning anything. Printing the value of the out parameter just before the procedure exits, the parameter's value is set and looks great. So why is my test code not getting any value back? ``` USE MyDB GO EXECUTE sp_addmessage @msgnum = 51001, @severity = 16, @msgtext = N'Resource NOT Available', @lang = 'us_english', @replace = REPLACE GO IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[GetNewFileNumber]') AND type in (N'P', N'PC')) DROP PROCEDURE [dbo].[GetNewFileNumber] GO CREATE PROCEDURE dbo.GetNewFileNumber ( @NextFileNum nvarchar(11) = NULL output ) AS BEGIN set transaction isolation level serializable begin transaction declare @LockResult int; declare @CurrentDate date; declare @FileNumberDate date; declare @FileNumber int; execute @LockResult = sp_getapplock @Resource = 'GetNewFileNumber_TRANSACTION', @LockMode = 'Exclusive', @LockTimeout = 0 if @LockResult <> 0 begin rollback transaction raiserror ( 51001, 16, 1 ) return end set @CurrentDate = CONVERT (date, GETUTCDATE()); select @FileNumberDate = filedate, @FileNumber = fileCount from dbo._globalCounters; if @FileNumberDate != @CurrentDate begin set @FileNumberDate = @CurrentDate; set @FileNumber = 0; end set @FileNumber = @FileNumber + 1; update dbo._globalCounters set fileDate = @FileNumberDate, fileCount = @FileNumber; set @NextFileNum = convert(nvarchar(6), @FileNumberDate, 12) + '-' + RIGHT('00'+convert(nvarchar(2), @FileNumber),2); execute sp_releaseapplock 'GetNewFileNumber_TRANSACTION' commit transaction print 'filenum:' + @NextFileNum return @FileNumber; END GO --Test the procedure DECLARE @return_value int DECLARE @out_value nvarchar(11) EXEC @return_value = [dbo].[GetNewFileNumber] @out_value SELECT 'Return Value' = @return_value, 'Out Value' = @out_value -- Out value always returns null?! GO ```