Sql-server – SELECT Data Set Using Stored Procedure with BCP Error

All, I have a BCP export query. It is failing with the useless error mesage on BCP usage:

usage: bcp {dbtable | query} {in | out | queryout | format} datafile
[-m maxerrors] [-f formatfile] [-e errfile]
[-F firstrow] [-L lastrow] [-b batchsize]
[-n native type] [-c character type] [-w wide character type]
[-N keep non-text native] [-V file format version] [-q quoted identifier]
[-C code page specifier] [-t field terminator] [-r row terminator]
[-i inputfile] [-o outfile] [-a packetsize]
[-S server name] [-U username] [-P password]
[-T trusted connection] [-v version] [-R regional enable]
[-k keep null values] [-E keep identity values]
[-h "load hints"] [-x generate xml format file]
[-d database name]
NULL

The query I am using is as follows

DECLARE @SQL VARCHAR(8000); 
SELECT @SQL = 'bcp "EXEC ispsSelectEmptyAsNull ''B1A'';" 
               queryout "F:\aaData\IPACostData\R15TData\2BSHAEOS_B1A_20121120.txt" 
               -f "F:\aaData\IPACostData\R15TData\tmpFormatCard_B1A.fmt" -T -S' + @@SERVERNAME + ''; 
EXEC xp_cmdshell @SQL;
GO

where if I replace EXEC ispsSelectEmptyAsNull ''B1A''; with SELECT * FROM B1A; there is not problem. The query EXEC ispsSelectEmptyAsNull ''B1A''; itself runs with no problems and returns the correct result set. I have run a million BCP queries but never using an SP to provide the result set. Am I doing this correct?

The SP is as follows:

IF EXISTS (SELECT name 
           FROM sys.procedures 
           WHERE name = N'ispsSelectEmptyAsNull') 
DROP PROCEDURE ispsSelectEmptyAsNull;
GO
CREATE PROCEDURE ispsSelectEmptyAsNull @TableName NVARCHAR(256)         
AS    
DECLARE @Columns VARCHAR(MAX);
SELECT @Columns =  
    COALESCE(@Columns + N', 
    NULLIF([' + CAST(COLUMN_NAME AS VARCHAR) + N'], '''') AS ' + COLUMN_NAME + '',
  N'NULLIF([' + CAST(COLUMN_NAME AS VARCHAR) + N'], '''') AS ' + COLUMN_NAME + '') 
FROM (SELECT DISTINCT COLUMN_NAME 
      FROM [IPACostAdmin].INFORMATION_SCHEMA.COLUMNS 
      WHERE TABLE_NAME = @TableName) AS H 
ORDER BY COLUMN_NAME; 
DECLARE @SQL NVARCHAR(MAX);
SET @SQL = N'
    SELECT ' + @Columns + N' 
    FROM ' + @TableName + N';';
-- SELECT CONVERT(XML, @SQL);
EXEC (@SQL);
GO

Thanks very much for your time.

Ps. I do not care about SQL injection etc.