Variable is undefined

[bfield]

I have used this line of coding multiple times. Sometimes it works and sometimes it doesn't. I cannot figure out what I am doing wrong to produce random results. Can I get some advice. 

My entire program is close to 100 pages so I am just posting a screen shot of the pertinent data.

[LucMeekes]

Considering Prime's error message: Are the labels the same?

Success!

Luc

[bfield]

Yes. They are identical. It is shown in the visual that they are.

[Werner_E]

What happens if you just evaluate the expression without assigning it to a variable?