Better error reporting when outer.concat fail

I encountered a strange behavior with inheritance and Properties items with outer.concat. Here is a minimal test code:

- TestA.qbs -
CppApplication {
    Depends { name: "cpp" }
    property bool testA: true
    Properties {
        condition: testA
        cpp.defines: ["TESTA"]
    }
}

- Project.qbs -
TestA {
    files: "main.cpp"
    Depends { name: "cpp" }
    Properties {
        condition: true
        cpp.defines: outer.concat(["TESTB"])
    }
}

With this code, qbs fail with the error : "Project.qbs ReferenceError: Can't find variable: testA"
Which is REALLY not helping...
If we change "outer" for "base" as it should be, no errors.

Would be nice to make the error message more helpful, but maybe there is a bug somewhere?