Is it ok to check legality of installing paid android app by checking getInstallerPackageName?

To ensure that my paid android application was legally installed from store, I write this:

String installer = getPackageManager().getInstallerPackageName(
        "com.example.myapp");

if (installer == null) {
    // app was illegally downloaded from unknown source. 
    // dear user, please re-install it from market
} 
else {
    // app was probably installed legally
    // (also it's good to check actual installer name)
}

Is it ok? Is there a chance that application that is legally purchased and installed from market will get empty installer package name and fail this test?

I understand that user can run adb -i com.fake.installer myapp.apk and pass this check, but it's more important if legal users will get potential problems or not.