Tri-state buffer feeds itself

I'm using Quartus II 9.0 Web Edition.

Verilog code:

...

input wr_in, rd_in;

inout [31:0] DATA, DATA_IN;

assign DATA = (!wr_in)? DATA_IN: 32'hzzzzzzzz;

assign DATA_IN = (!rd_in)? DATA: 32'hzzzzzzzz;

...

During compilation I'm getting error message:

Error: The tri-state buffer "ssram_cnt:inst1|DATA[31]" feeding the pin "data[31]" directly or indirectly feeds itself.

data[31] is I/O pin connected to DATA[31]

Not sure what's wrong with this code as I have seen the same example from Altera for CycloneIII starter board.

Any help would be highly appreciated.