Thx @Debaser : could you explain how I do the "check cell value with ISNUMBER" and "wrap formula in IFERROR" as I'm not to familiar with these aspects am afraid.
Thx for update @Debaser. Yes, it gives "zero/empty cell" result. This made me realise I included a wrong column (correct one has no letter) but IFERROR use is much appreciated for future reference. Thanks again.