[M::bwa_idx_load_from_disk] read 0 ALT contigs - empty sam files

[erah1]

I am trying to align a couple of paired-end sample fq.gz file using bwa mem, and I received the following error message.

[M::bwa_idx_load_from_disk] read 0 ALT contigs
orig.samal_2.fq.gz > /path/to/genome/genome_assembly.fasta 
/path/to/sample\
[M::bwa_idx_load_from_disk] read 0 ALT contigs
_R1.fq.gz'. open file /path/to/sample

This creates instantaneously an empty .sam file which name is sample.orig.sam, and the code stops running.

My command was:
bwa mem -t 32 -M ${refGen_fasta} sample_R1.fq.gz sample_R2.fq.gz > sample_orig.sam