Trimming commas from the end of a stringbuilder

Hello :)
I've a string builder, and I need to convert it to a string, and then trim any commas if there are any there (I need to get rid of them because I'm building an sql query).
Here's what I'm doing:
myStringBuilder.ToString().TrimEnd(','); 
Why isn't this working??? It's not trimming any commas at the end!
I'm using c# :)
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