Error message: Got a sequence without letters. Could not guess alphabet

[Alessandra-23]

Hi everyone,
I used the following command to run snippy 4.6.0-2:

snippy --cpus 16 --outdir snippy_output --ref reference_genome.fna --R1 ./N19_1paired.fq.gz --R2 ./N19_2paired.fq.gz mincov 10 --mapqual 60

However, during the generation of the snps.aligned.fa file, the following error message appears:
MSG: Got a sequence without letters. Could not guess alphabet.
Kindly could someone help me?

Best regards.
Alessandra from the University of Padua