What`S The Meaning Of N/A In Result Data Of Paml Dn/Ds Calculation?

Question

[harrymao20]

Detail: look into this result,some location(about 400bp),their dN/dS equal 0 . the result of dN/dS is N/A . this result means the input data was wrong or dN can be equal 0 ?

[Alastair Kerr]

how many synonymous and how many non-synonymous changes are present in these regions (those that have N/A)?

[Naren]

May be there are no 'synonymous substitutions' i.e. dS=0.

[Joseph Hughes]

Usually N/A stands for Not Available or Not Allowed. Did you get this in codeml?