Bowtie2 with no output

[superqqq]

Dear all:

I recently run Bowtie/2.2.0 on Linux

I used the command:

bowtie2 -x myindex -u ./myfastqfile --un ./161.fq -S 161_RNA.sam

I wants to store the unmapped reads in 161.fq and I got this error message:

Error: Encountered internal Bowtie 2 exception (#1)
Command: /opt/software/bowtie/2.2.0--GCC-4.4.5/bin/bowtie2-align --wrapper basic-0 -x rRNA_56 -U ./myfile --passthrough

Has anyone seen this before?

I am sure that the bowtie2 is run successfully and .sam file is generated, but no output file for unmapped reads, which I specified as 161.fq. Should I generate the empty file before I run this or it will automatically generate a file?

Thanks very much!