power - Xilinx Xpower Analyzer: Expected scope definition in VCD

I use a VCD file to evaluate the power of my design. The VCD is generated using the following command in the testbench file.

$dumpfile ("testb.vcd");
$dumpvars (0, tb.gcm_tb);

tb is the module name of the testbench. gcm_tb is the instantiated name of my design.

Xpower (version 10.1) gives me the following error:

ERROR:Power:976 - Parsing Error parse error line 7825 Expected scope definition here 

I notice that if I change the parameter into

$dumpvars (1, tb.gcm_tb);

then it does not give me the error. However, my design has several levels. Recording the power of only one level is not accurate.

How to solve this problem?