Probably misdiagnosed "Command textiota unavailable in encoding T1" error

I’m writing which is overall in English but with a few quotes with Greek characters in-between.
Now I’m getting a strange error saying ""Command textiota unavailable in encoding T1" (if you follow the indicated line, see snapshot below) which seems to be caused by a new Greek quote.
Before asking this question, I tried extracting a minimal version producing the error (see below) and the minimal version fails to reproduce the error, despite the two quotes being the same !
So it seems the real problem comes from elesewhere.

Here is the minimal example (that unfortunately fails to reproduce the error) :

documentclass[12pt,oneside]{book}
usepackage[a4paper,margin=1cm,nohead,foot=0.5cm]{geometry}
pagestyle{plain}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage[greek, english]{babel}
usepackage{lmodern}
usepackage{microtype}


begin{document}
footnotesize

This text is in English. And here is some Greek : foreignlanguage{greek}{μυστικὰ θεὰματα}

end{document}

And here is a snapshot of the error message :

documentclass[12pt]{article}
usepackage[greek, english]{babel}
usepackage[T1]{fontenc}

begin{document}
Ι I 
end{document}

! LaTeX Error: Command textIota unavailable in encoding T1.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.13 Ι       %<---------------- that is the problem
        I