Stored procedure doesn't return any row but same query returns proper data

Question

Here is my stored procedure:

CREATE PROC [SPmainReport]
@startDate date = null,
@endDate date = null,
@customerName varchar = null,

AS
SELECT Distinct
    VI.Code as CustomerCode, VI.Name as CustomerName, VI.Area as CustomerArea, VI.[Address] as [address], CP.ProductName as ProductName, CP.ProductQuantity as Quantity
from 
VendorTrading VT inner join CustomerProducts CP on VT.Id = CP.VendorTradingId inner join VendorInfo VI on VT.VendorId = VI.Id
where
 (VT.Tradedate between isnull(@startDate,VT.Tradedate) and isnull(@endDate,VT.Tradedate))
 and VI.Name = ISNULL(@customerName, VI.Name)

On execution it doesn't return any value but if I execute this query:

    SELECT Distinct
    VI.Code as CustomerCode, VI.Name as CustomerName, VI.Area as CustomerArea, VI.[Address] as [address], CP.ProductName as ProductName, CP.ProductQuantity as Quantity
from 
VendorTrading VT inner join CustomerProducts CP on VT.Id = CP.VendorTradingId inner join VendorInfo VI on VT.VendorId = VI.Id
where
 (VT.Tradedate between isnull(@startDate,VT.Tradedate) and isnull(@endDate,VT.Tradedate))
 and VI.Name = ISNULL('John', VI.Name)

It return exactly required data. I am totally confused why it is happening. There is no difference at all. I am making sure scripts are running on same database and it also contains perfect data. here is SP execution script:

USE [E:\SANDWICH3\ABC\BIN\DEBUG\DATABASE\ABC.MDF]
GO

DECLARE @return_value Int

EXEC    @return_value = [dbo].[SPmainReport]
    @startDate = '2015-12-25',
    @endDate = '2015-12-25',
    @customerName = N'John'

SELECT  @return_value as 'Return Value'

GO

One more strange I have noticed is that If I modify this SP and limit conditions with just DATES, not name. It works fine then. I am working on Visual Studio 2013 Interface of SQL server. Not management studio (In case it does matter)

Answer 1

In SQL Server, always use a length parameter when using varchar() and char() and related types:

CREATE PROCEDURE SPmainReport (
    @startDate date = null,
    @endDate date = null,
    @customerName varchar(255) = null
)
BEGIN
    . . . 
END;

Most customer names probably have more than one character.