List size mismatch errors occur when you try to use lists of different lengths together in a node that’s running in Loop Mode or expecting multiple list inputs. They’re easy to understand and fix once you know what to look for! This guide will help you identify these errors and show you exactly how to resolve them.
A list size mismatch error happens when a node receives multiple list inputs of different lengths. In Gumloop, when a node processes multiple list inputs together (eg in Loop Mode), the lists must be the same size so the node knows how to pair up the items.Let’s look at a simple example using a Combine Text node:
A list of 5 company names from a Google Sheet
A list of 2 company URLs from an Airtable
The node won’t know how to match up the remaining 3 companies, causing a list size mismatch error. Essentially, the Google Sheet list tells the node to loop 5 times while the Airtable list expects 2 iterations - this mismatch means inputs can’t be properly paired.
Let’s explore the two main scenarios where list size mismatches typically occur, using a real example workflow that processes company data. You can follow along with the example workflow here.
One input comes directly from a source (like a Google Sheet)
Another input goes through processing that may filter or skip items
The processed list ends up shorter than the source list
Real-World Example: Company Data ProcessingIn our example workflow, we’re:
Reading company URLs from a Google Sheet
Filtering invalid URLs
Scraping and summarizing company information
Combining the original company name with its description
Note: One of the inputs is not a valid URL
When we run this flow, we encounter a list size mismatch:
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Node 'Combine Text' running in List Mode has an input 'input2' which is a list of 4 items.This is not matching the size of another one of the inputs, which has 5 items.
This error occurs because:
The company names come directly from the Google Sheet (5 items)
The descriptions come from filtered and processed data (4 items, one was invalid)
The Combine Text node can’t match these different-sized lists
Using Subflows to Resolve List Size Mismatch Errors
The solution to these list size mismatches is proper flow organization using subflows. Let’s see how we can fix our example workflow.View the corrected workflow here
Key improvements in the solution:
Related operations (scraping, summarizing, text combination) are grouped in a subflow
Error Shield wraps the entire subflow
Failed items are handled consistently throughout the process
The key to resolving list size mismatches is understanding how Subflows & Error Shield affects your data when it’s placed in different locations. Let’s see why Error Shield works better around a subflow than around individual nodes.
Here’s the better approach:When Error Shield wraps a subflow:
If the scraper fails for 2 URLs
Error Shield removes those items from ALL operations in the subflow
Both the scraped content AND company names are removed for failed items
Result: Lists stay matched (both have 3 items)
Think of it this way: When Error Shield is around a subflow, it keeps related data together. If anything fails for an item, all data for that item is removed consistently. This prevents mismatches that happen when some data is removed in one place but kept in another.
Remember: If a node can fail for some items in a list, wrap its entire operation group (including any nodes that use related data) in a subflow with Error Shield.Learn more about subflows here: https://docs.gumloop.com/core-concepts/subflowsStill stuck? If you’ve tried these solutions and still can’t resolve your list size mismatch error, reach out on forum.gumloop.com and we’ll help!