tsort: "odd input of tokens" with paired input

[Alad Wenter]

Hello,

When trying to run tsort on a moderately large paired list, I get:

tsort: -: input contains an odd number of tokens

However, both columns have the same amount of entries:

awk '{print $1}' pre_tsort.txt | wc -l
637

awk '{print $2}' pre_tsort.txt | wc -l
637

The data in question (first column is a package, the second a 
dependency) is available here:

https://ptpb.pw/c1Vu

What am I missing?

Cheers,

Alad