Illegal reference to an array in awk (I am having trouble figuring out awk)

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http://stackoverflow.com – The script I am trying to use is: cat gatk_probes.interval_list | awk 'BEGIN{OFS="\t"; print "#CHR\tBP1\tBP2\tID"} {split($1,a,":"); chr=a[1]; if (match(chr,"chr")==0) {chr="chr"chr} split(a[2],b,"-"); bp1=b[1]; bp2=bp1; if (length(b) > 1) {bp2=b[2]} print chr,bp1,bp2,NR}' > ./EXOME.targets.reg I am getting the error: awk: line 1: illegal reference to array b Is there something obviously wrong? (HowTos)