The exporter/kindle/kindle_exporter.py is running with numerous warnings.

Warning(index build):W15008: language not supported. Using default phonetics for
spellchecker: english.
Warning(parser8):W26001: Index not supported for enhanced mobi.
Warning(prcgen):W14005: Missing value attribute on idx:iform tag. line:
0000014 in file: /tmp/mobi-rUe33d/OEBPS/Text/0_a.xhtml
Warning(prcgen):W14005: Missing value attribute on idx:iform tag. line:
0000044 in file: /tmp/mobi-rUe33d/OEBPS/Text/0_a.xhtml
Warning(prcgen):W14005: Missing value attribute on idx:iform tag. line:
0000084 in file: /tmp/mobi-rUe33d/OEBPS/Text/0_a.xhtml
Warning(prcgen):W14005: Missing value attribute on idx:iform tag. line:
0000116 in file: /tmp/mobi-rUe33d/OEBPS/Text/0_a.xhtml
...etc...