[android help] NoSuchFieldError on findViewById()

There's two android projects I want to merge, one is Main and the other is linked as Library. But I have some troubles on this line:

Button modificarC;
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.activity_menu_calificaciones);
modificarC = (Button) findViewById(R.id.btn_aseso); //HERE

I tried Project Clean... also I have android-support-v4.jar on my dependencies

And this is my trace:

04-24 19:39:52.568: E/AndroidRuntime(9486): FATAL EXCEPTION: main
04-24 19:39:52.568: E/AndroidRuntime(9486): java.lang.NoSuchFieldError: com.utez.sistemas.sam.R$id.btn_aseso
04-24 19:39:52.568: E/AndroidRuntime(9486):     at com.utez.sistemas.sam.calificaciones.CalificacionesActivity.onCreate(CalificacionesActivity.java:67)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at android.app.Activity.performCreate(Activity.java:4465)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1052)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1932)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1993)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at android.app.ActivityThread.access$600(ActivityThread.java:127)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1159)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at android.os.Looper.loop(Looper.java:137)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at android.app.ActivityThread.main(ActivityThread.java:4507)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at java.lang.reflect.Method.invokeNative(Native Method)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at java.lang.reflect.Method.invoke(Method.java:511)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
04-24 19:39:52.568: E/AndroidRuntime(9486):     at dalvik.system.NativeStart.main(Native Method)

Also here is the layout:

            style="@style/LoginFormContainer"
            android:layout_width="253dp"
            android:layout_height="260dp"
            android:layout_marginLeft="15dp"
            android:layout_marginTop="30dp"
            android:orientation="vertical" >

                            android:id="@+id/btn_alta"
                android:layout_width="match_parent"
                android:layout_height="63dp"
                android:layout_marginLeft="25dp"
                android:layout_marginTop="5dp"
                android:background="@drawable/btn_altacalif"
                android:contentDescription="@string/action_sign_in_register"
                android:paddingLeft="32dp"
                android:paddingRight="32dp"
                android:textAlignment="center" />

                            android:id="@+id/btn_aseso"
                android:layout_width="match_parent"
                android:layout_height="0dp"
                android:layout_marginLeft="25dp"
                android:layout_marginTop="5dp"
                android:layout_weight="0.16"
                android:background="@drawable/btn_modcalif"
                android:contentDescription="@string/action_sign_in_register"
                android:paddingLeft="32dp"
                android:paddingRight="32dp"
                android:textAlignment="center" />

        

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