query_posts() Variables as Arguments

The following works perfectly:

$current_issue = query_posts('tag=April+2012&category_name=Cover');

What I cannot get to work is:


$tag_var = 'April+2012';

$current_issue = query_posts("tag=$tag_var&category_name=Cover");

I’ve tried multiple variations such as:

$tag_var = 'April+2012';

$current_issue = query_posts('tag='.$tag_var.'&category_name=Cover');

But nothing works. I get either an empty array or the fisrst “n” posts (where “n” = the default posts-per-page setting)

Kudos and accolades for any help to make this work.