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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