Description:
I was defining names for cells in a spreadsheet and wanted to use a, b and c. While a and b were accepted, c produces an error message that it was an invalid name.
This not only happens with "c"; it also happens with "r". All other single letters (including æ, ø, å) were accepted.

Steps to Reproduce:
1.Select an arbitrary cell
2.Press Ctrl+F3
3.Select Add
4.Enter c in the name field
5.Delete c in the name field
6.Enter r in the name field
7.Delete r in the name field
8.Enter any other single letter in the name field

Actual Results:  
In steps 4 and 6 the error message "Invalid name. Only use letters, numbers and underscore." appears and the Add button is greyed (inactive).
In step 8 the Add button is activated.

Expected Results:
The software should accept a single c or r as a name just like all the other single letters.


Reproducible: Always

User Profile Reset: No

Additional Info:


User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:53.0) Gecko/20100101 Firefox/53.0