GroupByKey Problem Using Sheets as Key

Hi,

I am having an issue that I hope is simple to solve.

Context:

I have 2 sheets that have multiple revision clouds with comments that act as “version” numbers that I want to populate the title block with.

Sheet 1 Revision Cloud Comments:

00A
00B
00C

Sheet 2 Revision Cloud Comments:

00A

Problem:

I want to group these “comments” by sheet. So that the list structure looks like this:

List
| [0] List
0 00A
1 00B
2 00C
| [1] List
000A

I’d like to then choose the last comment in each sheet to populate the title block. However, I am getting an error “Warning: List.GroupByKey operation failed.
Number of items does not match the number of keys.”.

Any ideas on how I can achieve a list that is grouped by sheets?

Sample Revit Project and Dynamo file can be found at this link:
https://drive.google.com/drive/folders/0B8jARoP5hB9qUHpxbll1bm8yTHc?usp=sharing

Thanks in advance.