BIP default parameter issue.

Hi E
Could you please share me your advice on the following issue.
In BIP 11.1.1.7.150120 (build# 20150113.1211 on Tue Jan 13 12:15:48 JST 2015)
I created a data model,the parameter setting is as following.
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Parameter
Name       Data Type   Default Value  parameter Type
LP_KSHU    String                           menu
LP_LOT      String                            Text
LP_BF       Integer      20                  Text


LP_BF:Type:Text
Display Label    FOB buffer value
Text Field Size  2
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I found when open the report,clear the default value 20 at FOB buffer value parameter region,
then click apply button,the buffer value displayed as 20.0

when input NULL to FOB buffer value,how to make the report display as Null at FOB value