What does this "pu1" of the clk means in the modelsim-altera ?

What does this pu1 of the clk means in the modelsim-altera ?

The verilog codes of the cam in the cookbook (http://www.altera.com/literature/manual/stx_cookbook.pdf). There's 5 files:

ram_block.v

ram_based_cam_tb.v

ram_based_cam.v

cam_ram_block_tb.v

cam_ram_block.

All the files are compiled in the Quartus 11 successfully.The 2 testbench files (ram_based_cam_tb.v & cam_ram_block_tb.v) are also successful,but in the Modelsim 6.6 ,the waves are wrong, the "clk" is "pu1", what does this means? How to correct it ?