Android - button disappear from the layout when it has set color
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I have following Table in my Android app. Everything works, except the last button - about me. When I set background to transparent color - it is not beign display on my device (however, it is visible on the graphical view of the layout in the eclipse. By "not display" I mean, that it is not on the screen - on its place its "next". If I remove android:background from the last button - it comes back to device. Whats wrong?
<!-- language: lang-xml -->
    <TableLayout
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:orientation="horizontal"
                    android:stretchColumns="2" >
        
                    <TableRow
                        android:id="@+id/tableRow1"
                        android:layout_width="wrap_content"
                        android:layout_height="wrap_content"
                        android:orientation="horizontal" >
        
                        <ImageButton
                            android:id="@+id/buttonShare"
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:background="@color/transparent"
                            android:onClick="shareIt"
                            android:src="@drawable/social_share" />
        
                        <ImageButton
                            android:id="@+id/buttonBack"
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:background="@color/transparent"
                            android:onClick="prevImage"
                            android:src="@drawable/navigation_previous_item" />
        
                        <ImageButton
                            android:id="@+id/button1"
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:background="@color/transparent"
                            android:src="@drawable/navigation_accept" />
        
                        <ImageButton
                            android:id="@+id/buttonAbout"
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:background="@color/transparent"
                            android:onClick="showAbout"
                            android:src="@drawable/action_about" />
        
                        <ImageButton
                            android:id="@+id/buttonForward"
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:background="@color/transparent"
                            android:onClick="nextImage"
                            android:src="@drawable/navigation_next_item" />
        
                        <ImageButton
                            android:id="@+id/buttonAbout"
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:background="@color/transparent"
                            android:onClick="showAbout"
                            android:src="@drawable/action_about" />
                    </TableRow>
                </TableLayout>

Part of my color resource is like below:

    <color name="transparent">#00000000</color>
