Error: Can't place node "clkIn" -- node has differential I/O pin pair inconsistencies

Hi,

I am trying to assign my input clock an I/O standard of LVDS. and I get the following error:

'Can't place node "clkIn" -- node has differential I/O pin pair inconsistencies.'

Also, I get a critical warning just before the error, 'Unable to update property "I/O Standard" of node "clkIn".

Anyone have any idea?