Unclear error message «Assertion failed»

[MysaaJava]

When i use a predicate starting with a capital letter, elpi fails (expectedly), but the error message is really cryptic:

Fatal error: exception File "src/compiler/compiler.ml", line 989, characters 13-19: Assertion failed

Elpi could use a better error message

[gares]

Can you give me a test? Predicates can be capitals when the are variables, eg map calls a F on X to get a Y.