Unbound type error, what is it?

I’m exploring the Stable variables and upgrade methods :: Internet Computer example. When I use a record as value I got into this strange error that I don’t understand. What is unbound type caller error about?

import Principal "mo:base/Principal";
import Nat "mo:base/Nat";
import Nat32 "mo:base/Nat32";
import Hash "mo:base/Hash";
import HashMap "mo:base/HashMap";
import Text "mo:base/Text";
import Debug "mo:base/Debug";

type Record = { owner: Principal };

actor Registry {
    let records = HashMap.HashMap<Text, Record>(1024, Text.equal, Text.hash);

    public shared(msg) func set(name : Text) : async Text {
        let caller = msg.caller;
        records.put(name, { owner: caller; });  // <-- ERROR: type error [M0029], unbound type caller
        return ""
    };

    public func get(name: Text) : async ?Record {
        records.get(name)
    }
};

Colon always denotes a type annotation in Motoko. For
defining the fields in an object literal you want to use =, as in {owner = caller}.

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