Unbound value Error occurs when compiling locksmith on Ubuntu 20.04.

[tluio]

I compiled locksmith on a fresh installation of Ubuntu 20.04 as suggested:

sudo apt install gcc opam autoconf automake make gperf python indent emacs flex bison
git submodule update --init --recursive
opam init -n
opam switch create . 3.11.2
eval $(opam env)
./configure
make

But I got the following error:

make[1]: Entering directory '/home/ubuntu/Downloads/GitHub/locksmith/banshee'
make -C libcompat
make[2]: Entering directory '/home/ubuntu/Downloads/GitHub/locksmith/banshee/libcompat'
make[2]: Nothing to be done for 'all'.
make[2]: Leaving directory '/home/ubuntu/Downloads/GitHub/locksmith/banshee/libcompat'
make -C codegen
make[2]: Entering directory '/home/ubuntu/Downloads/GitHub/locksmith/banshee/codegen'
make[3]: Entering directory '/home/ubuntu/Downloads/GitHub/locksmith/banshee/codegen'
ocamlc -c spec_to_c.ml
File "spec_to_c.ml", line 129, characters 17-39:
Error: Unbound value String.uppercase_ascii
make[3]: *** [OcamlMakefile:868: spec_to_c.cmo] Error 2
make[3]: Leaving directory '/home/ubuntu/Downloads/GitHub/locksmith/banshee/codegen'
make[2]: *** [OcamlMakefile:630: byte-code] Error 2
make[2]: Leaving directory '/home/ubuntu/Downloads/GitHub/locksmith/banshee/codegen'
make[1]: *** [Makefile:47: codegen] Error 2
make[1]: Leaving directory '/home/ubuntu/Downloads/GitHub/locksmith/banshee'
make: *** [Makefile:47: all] Error 2

I am not familiar with Ocaml and do not know how to solve this problem.
Any feedback will be appreciate!