Excel 2007 :: Validation With ISTEXT Not Working When Number In Cell?

I'm trying to validate a cell in Excel 2007 that should contain only two letters.

Formula:
Code:
=IF(AND(LEN(A1)=2,ISTEXT(A1)),TRUE,FALSE)
When the value of A1 is JK, the result is TRUE
When the value of A1 is 12, the result is FALSE

When the value of A1 is 3K, the result is TRUE
When the value of A1 is K3, the result is TRUE

When the value of A1 is 123, the result is FALSE
When the value of A1 is JKL, the result is FALSE

When the value of A1 is 3, the result is FALSE
When the value of A1 is K, the result is FALSE

The LEN function works as expected, but the ISTEXT function does not, whenever the cell contains a number and a letter. I've tested the LEN and ISTEXT functions separately, and get the same results.

Is it me, or is it Excel?