ImageView won't fill parent

I have a ScrollView on one of my screens. I want the right edge to have a shadow. I decided the easiest way to do this was to make the child of the ScrollView a RelativeLayout, and have two children of the RelativeLayout -- one being a LinearLayout that will house the layout of the screen, and the second View being the shadow.

Like so...

<ScrollView
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:scrollbars="none" >

            <RelativeLayout
                android:layout_width="fill_parent"
                android:layout_height="wrap_content" >

                <LinearLayout
                    android:layout_width="fill_parent"
                    android:layout_height="wrap_content"
                    android:orientation="vertical" >
                <!-- stuff -->
                </LinearLayout>

                <ImageView
                    android:layout_width="wrap_content"
                    android:layout_height="fill_parent"
                    android:src="@drawable/shadow"
                    android:layout_alignParentRight="true"
                    />
            </RelativeLayout>
        </ScrollView>

Unfortunately, this doesn't quite work. The ImageView is forcing its dimensions to be the size of the image file. It will not stretch vertically to be the height of the RelativeLayout. I've also tried "match_parent" to no avail. The image is a 9-patch.

Ideas?