A value of type '...' cannot be added to a collection or dictionary of type 'UIElementCollection'

I am getting the following error when I am adding a custom control via XAML. What can be the possible reason?

A value of type '...' cannot be added to a collection or dictionary of type 'UIElementCollection'

<Grid x:Name="QuantityDetail" DataContext="{StaticResource ViewModel}">
    <GroupBox>
       .....
       <Label Style="{StaticResource ResourceKey=LabelValue}">Min</Label>
       <!-- The following control -->
       <NumericUpDown></NumericUpDown>
       .....
    </GroupBox>
</Grid>