I need to limit the characters of one cell to a maximum of 28.

Here's what I've done:

Data Validation
Allow: Text length
Data: Less than or equal to
Maximum: 28

However this isn't working. It is still allowing any number of characters
over 28. I even tried:

Allow: Text length
Data: Between
Minimum:0
Maximum: 28

all to no avail. Any ideas?