Resource id not found for attribute 'layout_weightSum' in package 'android'

I have this simple LinearLayout that displays fine in the graphical layout, but when I build the project, I get this console error (what am I missing here?):

Resource id not found for attribute 'layout_weightSum' in package 'android'

Here's the layout:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_weightSum="3"
    android:baselineAligned="false"
    android:orientation="horizontal" >

    <LinearLayout
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1" >

        <TextView
            android:id="@+id/tvSpine"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Spine"/>
    </LinearLayout>

    <LinearLayout
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1" >

        <TextView
            android:id="@+id/tvPage"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Page"/>
    </LinearLayout>

    <LinearLayout
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1" >

        <TextView
            android:id="@+id/tvThick"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Thick"/>
    </LinearLayout>
</LinearLayout>