App always stop when I change the image :     <android.support.design.widget.FloatingActionButton
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:clickable="true"
        app:fabSize="normal"
        app:srcCompat="@mipmap/ic_launcher"
        android:id="@+id/floatingActionButton"
        android:layout_gravity="bottom|end"
        android:layout_marginRight="20dp"
        android:layout_marginBottom="20dp"
        app:backgroundTint="#54ddde" />

Currently, floating button's image is android. But I want to change to 'plus' image.

-like this

    <android.support.design.widget.FloatingActionButton
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:clickable="true"
        app:fabSize="normal"
        android:src="@drawable/img_plus"
        android:id="@+id/floatingActionButton"
        android:layout_gravity="bottom|end"
        android:layout_marginRight="20dp"
        android:layout_marginBottom="20dp"
        app:backgroundTint="#54ddde" />

However I only change image, the app has stopped. I am really embarrassed.
help plz.