Gwt Binder problem.

Compile application shows the error. please help.

//environment

eclipse 3.5
gwt 2.4.0
app engine 1.5.3

//error

[ERROR] Element may only contain a single child element, but found
<g:VerticalPanel ui:field='panel1'> and <g:VerticalPanel
ui:field='panel2'>. Element <g:center>

//.xml file

<ui:UiBinder xmlns:ui='urn:ui:com.google.gwt.uibinder'
xmlns:g='urn:import:com.google.gwt.user.client.ui'>
<g:DockLayoutPanel>
<g:north size="25">
<g:FlowPanel>
<g:Label>Good</g:Label>
</g:FlowPanel>
</g:north>
<g:center>
<g:VerticalPanel ui:field="panel1">
<g:Button ui:field="button1">North</
g:Button>
<g:Button ui:field="button2">South</
g:Button>
</g:VerticalPanel>
<g:VerticalPanel ui:field="panel2">
<g:TextArea ui:field="greenText" />
</g:VerticalPanel>
</g:center>
</g:DockLayoutPanel>
</ui:UiBinder>

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