ModelSim XE III error

Hi all
i'm suing ISE 11.4 and ModelSim XE III when i do the behavioral simulation
it works well but when i do the post place & route simulation the ModelSim
report this errors
"# ** Error: netgen/par/fpu_double_timesim.vhd(45842): (vcom-1141)
Identifier "x_dsp48a1" does not identify a component declaration."
 
how can i fix this error?!
 
thanks in advance

	   
					
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