Reverse Polish Notation

[Imagine]

A tutorial that my teacher linked shows 9 7 - 2 / being invalid, but I don't see how it is.
My thought process:
After 9, 7 is read in, stack is:
7
9

- is read in, 9 and 7 is popped off, and 9 - 7 is pushed in:
2

The 2 after - is read in and pushed in:
2
2

/ is read in, leading to both 2's being popped off and 2/2 to be pushed in:
1

The tutorial shows that right after 9 -7 is calculated, two numbers are attempted to be popped off the stack, leading to an error because the stack currently only has 1 number in it. Am I doing something wrong?

[Locked]

It can also be 2 9 7 - / for it to work.

If you want to visualize it:

Input	Operation	Stack
2	Push Back	[2]
9	Push Back	[2,9]
7	Push Back	[2,9,7]
-	Subtract	[2,(9-7)]
-	Push Result	[2,2]
/	Divide	[(2/2)]
/	Push Result	[1]

Likewise, I think yours is still correct.

Input	Operation	Stack
9	Push Back	[9]
7	Push Back	[9,7]
-	Subtract	[9-7]
-	Push Result	[2]
2	Push Back	[2,2]
/	Divide	[2/2]
/	Push Result	[1]