Very odd?? Illegel mix of collations error

I have a table (depth as int, name as varchar).

If I run the following query I get NO ERROR
SELECT CONCAT(REPEAT(' ', 2) , display_name) as name
FROM test

If I run this:
SELECT CONCAT(REPEAT(' ', depth) , display_name) as name
FROM test

I get the following error:

Illegal mix of collations (utf8_general_ci, COERCIBLE) and
(latin1_swedish_ci, IMPLICIT) for operation concat.

Can anyone explain this to me. I can't find an explanation anywhere.
Thanks