Unrecognized element 'add' error when reading web.config

When I do this `ConfigurationManager.GetSection("SectionA/sectionD")` I receive this error :

    Unrecognized element 'add'

I'd like read all "add" element from this section to create a collection.

    <configSections>
    	<sectionGroup name="SectionA">
    		<sectionGroup name="SectionB">
    			<section name="sectionC" type="MyProject.SectionC,MyProject" />
    			<section name="sectionD" type="MyProject.SectionD,MyProject" />
    		</sectionGroup>
    		<section name="sectionE" type="MyProject.SectionE,MyProject" />
    	</sectionGroup>
    </configSections>
    
    <SectionA>
    	<SectionB>
    		<sectionD>
    			<add key="PerPage10" value="10" />
    			<add key="PerPage30" value="30" />
    			<add key="PerPage60" value="60" />
    		</sectionD>
    		<sectionC attribute3="10" />
    	</SectionB>
    	<sectionE attribute1="3" attribute2="5" />
    </SectionA>



Unrecognized element 'add' error when reading web.config
===


When I do this `ConfigurationManager.GetSection("SectionA/sectionD")` I receive this error :

    Unrecognized element 'add'

I'd like read all "add" element from this section to create a collection.

    <configSections>
    	<sectionGroup name="SectionA">
    		<sectionGroup name="SectionB">
    			<section name="sectionC" type="MyProject.SectionC,MyProject" />
    			<section name="sectionD" type="MyProject.SectionD,MyProject" />
    		</sectionGroup>
    		<section name="sectionE" type="MyProject.SectionE,MyProject" />
    	</sectionGroup>
    </configSections>
    
    <SectionA>
    	<SectionB>
    		<sectionD>
    			<add key="PerPage10" value="10" />
    			<add key="PerPage30" value="30" />
    			<add key="PerPage60" value="60" />
    		</sectionD>
    		<sectionC attribute3="10" />
    	</SectionB>
    	<sectionE attribute1="3" attribute2="5" />
    </SectionA>


