Skip to content

Cannot get property id on null object

Issue

Console

Execution aborted due to an unexpected error

 -- Check script '/home/jfellows/Documents/git/nf-core/taxprofiler/./workflows/../modules/nf-core/modules/kraken2/kraken2/main.nf' at line: 2 or see '.nextflow.log' file for more details

Log

java.lang.NullPointerException: Cannot get property 'id' on null object

Possible source

In most cases meta.id does exist, it is just not accessible by Nextflow in the way it expects to.

  • Input channels not correctly passed to a module
    • meta tuple is incorrectly nested (e.g. [[<meta>]] vs. [<meta>])
    • Other parts of an input channel are incorrectly nested e.g. [meta] [[reads1.fq, reads2.fq]] when should be [meta] [reads1.fq, reads2.fq]
  • Two separate input channels both have a meta.id tag (i.e., Nextflow doesn't know which one to take).
  • Can also occur when meta.id tag is specified the tag block of a module, but meta is not supplied as input.

Recommendation

Use static typing with Nextflow 26.10 or later, and declare the process input as a record or a typed tuple. nextflow lint then reports a wrongly nested meta map or reads list at the line where the process is called. It also reports meta used in a process that has no meta input, including in directives such as tag.

Without static typing, Nextflow 25.04 or later reports it in the output: and script: blocks, but not in directives.