enableCompilerDefinesByLanguage is unknown at first start

Project {
    Product {
        Depends { name: "cpp" }
    
        cpp.enableCompilerDefinesByLanguage: {
            console.warn(typeof base);
            return base;
        }
    }
} 

$ qbs profile:glibc-x86_64-gcc11
Restoring build graph from disk Project file '/Users/lo1ol/Documents/work/unilogon/rtlogon.qbs' has changed.
Resolving project for configuration default
WARNING: unknown
WARNING: object
Build done for configuration default.
Building for configuration default 

cpp.enableCompilerDefinesByLanguage: base.concat(qbs.toolchain.contains("clang-cl") ? ["c"] : [])

cpp.enableCompilerDefinesByLanguage: {
    if (typeof base === "unknown")
       return base;
    return base.concat(qbs.toolchain.contains("clang-cl") ? ["c"] : []);
}