Pjiang, Xiaobaidu need your advice

Pjiang, Xiaobaidu and other friends,

I submitted NIW EB2 application including I-140 for me and I-485 for whole family in september 2005. My I-140 was approved in Oct 2005. Due to name check, our I-485 are still pending. Today I received a letter from USCIS asking me to submit I-864 form. Should I do it? Our applications are employment-based EB2. I-864 is not necessary. It seems that the letter does not make any sense. Thanks for your response.