index of an array is not recognized in awk

I have two tab separated files, each with two columns. I want to create a file which contains overlapping elements by column 1 of the two files. To do so, I put file 1 in an array first then scanned the array to check against file 2 for overlaps. However, somehow the index of the array cannot be recognized. See below for the elaboration of the problem. The first 3 lines of the files look like this: File 1: 90001 raw acceleration data 2634 Heavy DIY 1011 Light DIY File 2: 2634 218263 25680 44313 25681 44313 To show that there are overlaps in column 1 of the two files: user@cluster:~> grep 90001 file2 90001 103662 user@cluster:~> grep 2634 file2 2634 218263 To create file 3, I tried this first, which yielded an empty file. awk 'BEGIN {FS = "\t"; OFS= "\t"} NR==FNR {a[$1]=$2; next} { if($1 in a) print $1, a[$1]}' file1 file2 > file3 The following code confirmed the issue is the index of the array was not recognized; because adding the else line actually prints file2 into file3. awk 'BEGIN {FS = "\t"; OFS= "\t"} NR==FNR {a[$1]=$2; next} {if($1 in a) print $1, a[$1] else print $1, $2}' file1 file2 > file3 I am quite puzzled. I wonder what might have caused the issue and how I can fix it? Thanks in advance.