'Undefined Index: Key' when using a files field in a nested builder

[baraglobal]

Kirby: 3.5.6 Builder: 2.0.16

When using nested builders, any effort to use a files field results in an error 'Undefined Index: [field key]'. It's not something I've encountered in the past so having some trouble pinpointing what could be causing it. Any help would be massively appreciated!

Example code:

    descriptionBuilder:
      label: Description Builder
      type: builder
      columns: 1
      fieldsets:

        descriptionBlock:
          name: Description Block
          label: '{{ model }} wears...'
          fields:

            model:
              label: Model Name
              type: text  

            garmentBuilder:
              label: Garment Builder
              type: builder
              columns: 1
              fieldsets:

                garment:
                  name: Garment
                  label: Garment
                  fields:

                    garmentName:
                      label: Garment Name
                      type: text

                    garmentImage:
                      label: Garment Image
                      type: files
                      max: 1

[TimOetting]

Closing this issue because this plugin will no longer be maintained, as its main functionality can be replaced by Kirby's built-in Blocks Field and Layout Field