snakemake snakefile中的语法错误

我试着运行snakemake来测试一个小任务。代码如下:

rule kallisto_quant:
    input:
        idx='/fullpath/snakemake-example/Kallisto_test/Arabidopsis_thaliana.fa.index'
        fwd='/fullpath/snakemake-example/Kallisto_test/Condition1_R1_008.trimmed.fastq.gz'
        rvs='/fullpath/snakemake-example/Kallisto_test/Condition1_R2_008.trimmed.fastq.gz'
    output:
        '/Condition1'
    threads: 10
    shell:
        'kallisto quant -i {input.idx} -o {output} -b 100 {input.fwd} {input.rvs}'

运行此命令时,会出现语法错误:

SyntaxError in line 4 of /fullpath/snakemake-example/Snakefile:
invalid syntax

参考snakemake手册,我看不到任何语法错误。这里会有什么问题?

提前谢谢。