Chlorine reading on Test Strip - only Rain Water in tank

Hayls23

I have carried out a JBL water test strip - and it has come back with a medium to high reading of chlorine in the water. The problem is there isn't any chlorine in the water. For our water changes in our aquariums we only ever use rain water - from a water tank used solely for this purpose.

How do I work out why this reading would be present on the test strip.

Thanks