SQL Server drop default constraint

DECLARE @defname VARCHAR(100), @cmd VARCHAR(1000)
SET @defname =
   (SELECT name FROM sysobjects so
     JOIN sysconstraints sc ON so.id = sc.constid
   WHERE object_name(so.parent_obj) = 'TableName'
     AND sc.colid =
         (SELECT colid FROM syscolumns WHERE id = object_id('dbo.TableName') AND name = 'FieldName'))
    SET @cmd = 'ALTER TABLE TableName DROP CONSTRAINT ' + @defname
   EXEC(@cmd)
GO

(http://stackoverflow.com/questions/3136866/sql-server-drop-default-syntax-error)