Using Retool to Update Dynamodb Item - ExpressionAttributeValues contains invalid key: Syntax error; key: "44" id

Question

I'm using the Dynamodb resource in Retool, which is successful for GETs/Scans/Puts/Queries, but I can't seem to get an UpdateItem statement to work.

I'm trying to update an item to add a key for a list of maps if it doesn't exist and append an item if the key already does exist.

Configuration

Update Expression

SET images = list_append(:val, if_not_exists(images, :emptylist))

ExpressionAttributeValues

In Retool, my ExpressionAttributeValues are ":val": [{"location": "{{s3Uploader1.s3FolderName}}/{{s3Uploader1.s3FileName}}"}], ":emptylist":[], which pulls the s3 folder and file names from an s3Uploader and renders to ":val": [{"location": "redactedpath/redacted/redactedfilename"}], ":emptylist":[]

I originally tried the format of calling out the data types, e.g. "M", "L", etc, but I got exactly the same error.

  ":val": 
 {
   "L":
   [
     {
       "M": 
         {
           "location":
           {
         "S": "{{s3Uploader1.s3FolderName}}/{{s3Uploader1.s3FileName}}"
           } 
         }
     }
   ]
 }, 
":emptylist":
   {
     "L":[]
   } 

Result/Error

When I run the query, I get the following error:

statusCode:422
error:"Unprocessable Entity"
message:"ExpressionAttributeValues contains invalid key: Syntax error; key: "44""
data:null
estimatedResponseSizeBytes:147
resourceTimeTakenMs:363
isPreview:false
resourceType:"dynamodb"
lastReceivedFromResourceAt:1644774304601
source:"resource"

From my understanding, that error message usually specifies the actual key that caused the problem, but from what I can tell, my ExpressionAttributeValues does not contain the string 44. I'm wondering if this is something coming from Retool or if it's perhaps a location instead of the actual key.

I've dug through what feels like the depths of StackOverflow to try different things, but now I feel like I'm stuck.

Additional Information

• My original ExpressionAttributeValues was based on Is it possible to combine if_not_exists and list_append in update_item
• Similar question, but no answer and different key: ValidationException: ExpressionAttributeValues contains invalid key

Is there anything in the ExpressionAttributeValues that looks like it could cause that error?