DynamoDB API filtered scan returns no results

So I'm running a scan of a DynamoDB table with ~500 records using the AWS CLI through Powershell, because the AWS Powershell Tools don't support DDB query/scan operations. I can run the command with no filters and get all my items:

& aws dynamodb scan --table-name "$table_name" --projection-expression "$item_key"

This returns all 500+ values of $item_key.

Problem comes when I try to filter the scan:

& aws dynamodb scan --table-name "$table_name" --projection-expression "$item_key" --filter-expression "item_key_2 = `"$item_value`""

This returns a count of 0 and no items, even though there are several values in the table that match $item_value.

What am I missing/doing wrong here?