unrecognised objects in Quartus

hi everyone,

i'm experimenting with my DE1 board and i've come across something strange. When coding (VHDL) I first use modelsim for simulation and syntax checking. Then the code goes straight trough quartus for mapping to hardware.

I've made this program and modelsim compiles it without complaining, but with quartus (8.1, free edition) i get this response:

Warning (10445): VHDL Subtype or Type Declaration warning at gol.vhd(323): subtype or type has null range

Error (10344): VHDL expression error at gol.vhd(323): expression has 4 elements, but must have 8 elements

Error: Can't elaborate top-level user hierarchy

Error: Quartus II Analysis & Synthesis was unsuccessful. 2 errors, 1 warning

And the specific line is:

neighbours := "000" & prev_obj_reg(word_counter)(obj_counter DOWNTO obj_counter+1) & obj_reg(word_counter)(obj_counter+1) & next_obj_reg(word_counter)(obj_counter DOWNTO obj_counter+1);

With this declaration:

TYPE registerType IS ARRAY (INTEGER RANGE 0 TO 6) OF STD_LOGIC_VECTOR(15 DOWNTO 0);

SIGNAL obj_reg,prev_obj_reg, next_obj_reg : registerType;

VARIABLE neighbours : STD_LOGIC_VECTOR (7 DOWNTO 0);

When I delete the "000" I get the error "Error (10344): VHDL expression error at gol.vhd(323): expression has 1 elements, but must have 8 elements", so Quartus does not seem to recognise the concatenated elements from the registers.

Please help me fix this