What is wrong with my Add6 chip? nandtotertis

CHIP Add4 { IN a[4], b[4], c; OUT out[4],carry; PARTS: FullAdder(a=a[0],b=b[0], c=c, sum=out[0], carry=carrylvl1); FullAdder(a=a[1],b=b[1], c=carrylvl1, sum=out[1], carry=carrylvl2); FullAdder(a=a[2],b=b[2], c=carrylvl2, sum=out[2], carry=carrylvl3); FullAdder(a=a[3],b=b[3], c=carrylvl3, sum=out[3], carry=carry); }

CHIP Add6 { IN a[6], b[6]; OUT out[6],carry; PARTS: Add4(a=a[0..3], b=b[0..3], c=false, out=out[0..3] , carry=c1); Add4(a=a[4..7], b=b[4..7], c=c1, out=out[4..7], carry=carry); }

my add6 is supposed to Add two 6-bit values out=a+b with both inputs a and b are unsigned carry output should reflect the overflow of the Add6 chip i get the error a[4..7]: the specified sub bus is not in the bus range how can i fix this?