"offending character" warnings

[AledHe]

...
[2024-06-04 21:44:51] : WARNING : offending character: >>>X<<< (/kalign-main/lib/src/msa_op.c line 356)
[2024-06-04 21:44:51] : WARNING : there should be no character not matching the alphabet (/kalign-main/lib/src/msa_op.c line 355)
[2024-06-04 21:44:51] : WARNING : offending character: >>>Q<<< (/kalign-main/lib/src/msa_op.c line 356)
[2024-06-04 21:44:51] : WARNING : there should be no character not matching the alphabet (/kalign-main/lib/src/msa_op.c line 355)
...

Get above warnings when executing Kalign by include its libs.

What invoked this type of error?

is it something related to the input dataset alphabet?