AutoCAD Inventor :: Changing Extrusion By Changing Sketch Results In Errors

The attached files are two sprockets. The original part has 30 teeth. I need 34 teeth.

The MODIFIED part has one of my attempts at making 34 teeth. I have tried the following with the same error dialog box (see below):

1. Edit Sketch_6 (associated with Extrusion1) by deleting all but one of the teeth lines (2 arcs and 2 lines that make up a tooth) and making a circular pattern of 34.

2. Same circular pattern, except this time I "Close the Loop" (right-click on one of the teeth line segments > Close Loop > click on all the segments until the loop is closed).

3. Same circular pattern, except this time change the resulting patterned lines into construction lines, trace these lines with arcs and lines of my own (not construction lines), and close the loop on my own lines.

When each of the 3 attempts failed, I tried deleting Extrusion1 and making a new Extrusion feature from Sketch_6. I cannot select what I want in Sketch_6 in order to make the teeth.

I'm at the point where I'm just going to take the dimensions from the circular pattern sketch of 34 teeth, delete Extrusion1 and Sketch_6, and make my own sketch and extrusion.

For the future, I really would like to know - why is it so difficult to alter features like this? Yes, I understand that Sketch_6 is not fully constrained, and on top of this I am not privy to how it was generated in the first place (this is a part file from a vendor). But it does not strike me as something overly complicated for the Extrusion feature to recognize a change in its associated sketch. This is not the first time I have encountered something like this.