DynamoDb Between Query always returns null

Question

I'm trying to return the results where a row is equal to the first attribute, and falls between a date range for the second attribute. This always returns no results, even though I can see valid results in the table. I can't find a lot of documentation on using the Between operator, does that look incorrrect?

       var filter = new QueryFilter();
            filter.AddCondition(primaryAttribute, QueryOperator.Equal, primaryValueToSearch);
            filter.AddCondition(secondaryAttribute, QueryOperator.Between, rangeBegin, rangeEnd);
            QueryOperationConfig config = new QueryOperationConfig{IndexName = primaryAttribute+"-"+secondaryAttribute+"-index", Filter = filter};
            var result = await _dynamoDbContext.FromQueryAsync<TracedRecord>(config, _dynamoDbOperationConfig).GetRemainingAsync();
            return result;
        }