Reports :: Access 2007 Trim And Field Text Addition

I created a form letter as an Access 2007 report. I want the greeting to read, "Dear [first_name]," e.g., "Dear Alan,". The [first_name] field is bound to said field in a query. When I just use the [first_name] field, I get "Dear Alan" with no comma. I've tried to add the comma various ways:

"=Trim[first_name] & ",", "=[first_name] & ,", "=Trim([first_name] & ","), etc., etc.

Any addition to the basic field produces the "#Type!" error. I also created labels from the same query using

"=Trim([first_name] & " " & [last_name])",

and that works fine.