---
title: "DSL2 Common Nextflow 'uninformative' errors & possible solutions"
tags: open-source, nf-core, Nextflow, DSL2, best practices, coding, guidelines, common-errors
---

# Common Nextflow 'unformative' errors & possible solutions

- [James A. Fellows Yates](https://github.com/jfy133)
- [Simon Pearce](https://github.com/sppearce)
- [Friederike Hanssen](https://github.com/FriederikeHanssen)

Despite being very powerful, Nextflow has a slightly 'infamous' habit of giving extremely generic and unclear errors that are difficult to to traceback to what the cause is.

This document is an effort to collect common errors that often trip up developers, and record possible solutions.

## Missing process or function with name 'dump'

### Issue

When using the `.dump()` function you get an error to console

Missing process or function with name 'dump'

### Possible Error Source

In many cases, the function is not _missing_, just incorrectly applied.

- Missing `tag:` before the actual tag name (e.g. `.dump('my tag') vs .dump(tag: 'my_tag')`)
- Placed on a channel-type that does not support the `.dump` function (e.g. channels with branches)
- `.dump()` was applied on a multi-channel output object without specifying which channel

## 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 Error 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]`
- Can also occur when `meta.id` tag is specified the `tag` block of a module, but `meta` is not supplied as input.

## Missing process or function with name 'mix'

### Issue

You get the follow error when using `.mix()` on channels

Missing process or function with name 'mix'

### Possible Error Source

- You passed a multi-channel output variable to `.mix`
- e.g. this will cause the error:

    ch_scaffolds2bin_for_dastool
        .mix(DASTOOL_SCAFFOLDS2BIN_METABAT2.out.scaffolds2bin)
        .mix(DASTOOL_SCAFFOLDS2BIN_MAXBIN2)

when you should've specified the specific `.out` channels

    ch_scaffolds2bin_for_dastool
        .mix(DASTOOL_SCAFFOLDS2BIN_METABAT2.out.scaffolds2bin)
        .mix(DASTOOL_SCAFFOLDS2BIN_MAXBIN2.out.scaffolds2bin)

## Module compilation error

### Issue

You get the following error, connected to the whole workflow

 - file : [PATH]
 -  cause: Unexpected input: '{' @ line N, column N.
 -  workflow [WORKFLOW] {

### Possible Error Source

- You put two `.` next to each other, such as at the end of the first line and the start of the second:

PROCESS.
.out
.bam
.set{ch_bams}

## DataVariable can only be assigned once

### Issue

You get the following error, connected to a process

Error executing process > 'PROCESS'

Caused by:
  A DataflowVariable can only be assigned once. Only re-assignments to an equal value are allowed.

### Possible Error Source

You forgot the brackets around a function like `flatten()`:

PROCESS
.out
.vcfs
.flatten

## Multi-channel output cannot be applied to operator <x> for which argument is already provided

### Issue

You get an error such as

Multi-channel output cannot be applied to operator mix for which argument is already provided

### Possible Error Source

You likely forgot to specify the output channel of a multi-channel output module or subworkflow

Where

ch_output = MODULE_A ( input )

MODULE_B ( ch_output )

Should be

ch_output = MODULE_A ( input ).reads

MODULE_B ( ch_output )

## Unable to parse config file

### Issue

    Unable to parse config file: 'nextflow.config'

  Compile failed for sources FixedSetSources[name='/groovy/script/Script7452FB2E4729BDF4899A4D4633CAE72A']. Cause: org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
  /groovy/script/Script7452FB2E4729BDF4899A4D4633CAE72A: 432: Unexpected input: '{' @ line 432, column 8.
     process{
            ^

### Possible Error Source

Syntax error in `modules.config`, such as:

1. Missing coma in `publishDir`
2. Additional `=` after ext.<> directive
3. Missing `{` or `}`somewhere

Mentioned line numbers OR mentioned sign are not indicative of where to search for the error, i.e. in the above example the actual problem was a duplicated `=`.