Error due to always enabled I/O buffer

Hi all,

I had generated a SOPC system using sdram controller, jtag, 2 pio and nios. When I compile the design, it shows error as shown below:

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Error: The node "try:NiosII|sdram_0:the_sdram_0|za_data[11]" has multiple drivers due to the always-enabled I/O buffer "try:NiosII|zs_dq_to_and_from_the_sdram_0[11]"

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try is the SOPC system name. For the sdram, I had also connected pll for clock skew. The error is not for za_data[11] only but also for [1] to [15]. Please help. I thought generated SOPC would not have any problem in compilation but guess I am wrong.