[python G2P] Error while processing word 'ä', skipping char 'ä'. There may be errors at phoneme output

I'm having the following error

[cassio@ciro12 12:06] fb-nlp/nlp-generator(master) $ python3 FalaBrasilNLP.py fb_nlplib.jar oi
DEMO: "oi"
[VOW] #ä# stress vowel not found!
[G2P] Error while processing word 'ä', skipping char 'ä'. There may be errors at phoneme output
  g2p:       
[VOW] #ä# stress vowel not found!
  syll:      
[VOW] #ä# stress vowel not found!
  stress:    0
[VOW] #ä# stress vowel not found!
[VOW] #ä# stress vowel not found!
[G2P] Error while processing word 'ä', skipping char 'ä'. There may be errors at phoneme output
[VOW] #ä# stress vowel not found!
  syl pohns: 

OS specs

[cassio@ciro12 12:35] fb-nlp/nlp-generator(master) $ neofetch 
                   -`                    cassio@ciro12 
                  .o+`                   ------------- 
                 `ooo/                   OS: Arch Linux x86_64 
                `+oooo:                  Host: Inspiron 3442 
               `+oooooo:                 Kernel: 5.3.1-arch1-1-ARCH 
               -+oooooo+:                Uptime: 51 mins 
             `/:-:++oooo+:               Packages: 1039 (pacman) 
            `/++++/+++++++:              Shell: bash 5.0.11 
           `/++++++++++++++:             Resolution: 1366x768 
          `/+++ooooooooooooo/`           DE: Cinnamon 4.2.4 
         ./ooosssso++osssssso+`          Theme: Adwaita [GTK2/3] 
        .oossssso-````/ossssss+`         Icons: Adwaita [GTK2/3] 
       -osssssso.      :ssssssso.        Terminal: yakuake 
      :osssssss/        osssso+++.       Terminal Font: Source Code Variable 9 
     /ossssssss/        +ssssooo/-       CPU: Intel i5-4210U (4) @ 2.700GHz 
   `/ossssso+/:-        -:/+osssso+-     GPU: NVIDIA GeForce 610M/710M/810M/820M / GT 620M/625M/630M/720M 
  `+sso+:-`                 `.-/+oso:    GPU: Intel Haswell-ULT 
 `++:.                           `-/+/   Memory: 3891MiB / 7867MiB 
 .`                                 `/