error "Not the start of the UTF-8 sequence"

[spir]

Hello,

I get this error message:
        Not the start of the UTF-8 sequence
without any other comment module name or whatnot.

This happens when I just added toString to the following struct, and used it:

    struct Node {
        // Note: level is equal to the number of chars up to this node.
        // Note: a 'path' node without entry has key="".
        char ch;
        uint level;
        Key key;
        Value value;
        ...
        string toString() {
            if (this.key != "")
                return format("<%s %s:%s>", this.ch, this.key, this.value);
            else
                return format("<%s>", this.ch);
        }
    }

Hints? I'm blocked :-(

Denis
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