Unexpected warning when appending empty label with +swap

[ftrebien]

ImageMagick version

6.9.11-60

Operating system

Linux

Operating system, version and so on

Debian 12 (Bookworm)

Description

When running convert-im6 with label:'' (empty label) followed by the +swap operator, ImageMagick succeeds with an unexpected warning.

Steps to Reproduce

1. Run the following command:

   convert-im6 /path/to/input.png label:'' +swap -append /tmp/output.png

   Result: the command completes successfully but generates the following warning:

   convert-im6.q16: no such image '/path/to/input.png' @ error/mogrify.c/MogrifyImageList/8802.
   

2. Run the same command with a non-empty label:

   convert-im6 /path/to/input.png label:'a' +swap -append /tmp/output.png

   Result: success and no warning.

3. Run the same command without +swap:

   convert-im6 /path/to/input.png label:'' -append /tmp/output.png

   Result: success and no warning.

Images

The error seems to be independent of input. The following command also issues a warning:

convert-im6 -size 100x100 xc:white label:'' +swap -append /tmp/output.png

[fmw42]

I get the same message on IM 6.9.13-19

convert lena.png label:'' +swap -append xyz.png
convert: no such image `lena.png' @ error/mogrify.c/MogrifyImageList/8836.

[urban-warrior]

label:'' does not return an image and +swap requires an image sequence. The no such image exception is misleading so we added patch to return invalid image index instead, the same exception you can expect from IMv7.