diff --git a/lib/CPUs/MIPS/src/core/mips_shifter.vhd b/lib/CPUs/MIPS/src/core/mips_shifter.vhd
index 34036a2..7f047ec 100644
--- a/lib/CPUs/MIPS/src/core/mips_shifter.vhd
+++ b/lib/CPUs/MIPS/src/core/mips_shifter.vhd
@@ -54,10 +54,12 @@ architecture Behavioral of shifter is
 function gen_fill_mask_rom(data_width : natural) return fill_mask_rom_t is
 	variable result : fill_mask_rom_t;
 	begin
-		for i in 0 to data_width-1 loop
+	    result(0) := (data_width-1 downto 0 => '0');
+		for i in 1 to data_width-1 loop
 			result(i) := (data_width-1-i downto 0 => '0') & (i-1 downto 0 => '1');
 		end loop;	
-		for i in 0 to data_width-1 loop
+	    result(data_width) := (data_width-1 downto 0 => '0');
+		for i in 1 to data_width-1 loop
 			result(data_width+i) := (i-1 downto 0 => '1') & (data_width-1-i downto 0 => '0');
 		end loop;	
 	return result;