请教下ERROR:MapLib:978这是个什么错误？

在用ISE12.3综合时。在MAP过程中出现下面错误，这个错误是什么意思啊，不太明白啊？
ERROR:MapLib:978 - LUT3 symbol "U5/BU2/U0/grf.rf/mem/gbm.tmp_ram_rd_en1" (output
   signal=U5/BU2/U0/grf.rf/mem/gbm.tmp_ram_rd_en) has an equation that uses
   input pin I2, which no longer has a connected signal. Please ensure that all
   the pins used in the equation for this LUT have signals that are not trimmed
   (see Section 5 of the Map Report File for details on which signals were
   trimmed)