为什么regMsg总是NULL

CREATE PROCEDURE pr
  @AVC_RetMsg   VARCHAR(255) OUT,
  @AN_funID     INT               =-1
AS
BEGIN
  ...
  ...
  SET @AVC_RetMsg = '操作成功！'
  PRINT @AVC_RetMsg
  RETURN 0
errRet:
  RETURN -1
END调用：
DECLARE @AVC_RetMsg VARCHAR(255)
EXEC pr @AVC_RetMsg,90101
SELECT @AVC_RetMsg retMsg为什么regMsg总是NULL