XST Warning 790: What does it mean?

[agatha14]

Hello to all of you,
I'm new to VHDL and I'm trying to understand how it works but I get to have a little problem. I'm trying to make the data memory for a processor. The ADDRESS consists of 32 bits but only 12 of them are used. When I write this code:

addr := ADDRESS(11 downto 0) ;
MEMORY(TO_INTEGER(UNSIGNED(addr))) <= DATA_W(7 downto 0);

everything is all right. But when I try to align the address and write:

addr := ADDRESS(11 downto 2) & "00" ;
MEMORY( TO_INTEGER(UNSIGNED(addr))) <= DATA_W(7 downto 0);

I keep getting this warning:
Warning: 790: Index value does not match array range, simulation mismatch

Can anyone explain what happens and it doesn't work?
Thank you in advance