xsd doesn't work with nullable value

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C#

XML

XSD

hello,
i have a problem to validate my Xml file with xsd in c# win application.
i have customize my element xsd with nillable="true" but doesn't work and gives me error:

The element 'FLG_SUPERVISOR' is not valid. value '' is not valid for the data type 'http://www.w3.org/2001/XMLSchema:decimal' - The string \" \ "is not a valid Decimal value

my xsd is this:

<?xml version="1.0" encoding="Windows-1252"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified"

    xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="DocumentElement">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" name="UserSettings" >
          <xs:complexType>
            <xs:sequence>
              <xs:element name="ID_UTENTE" minOccurs="1" maxOccurs="1" type="xs:decimal" />
              <xs:element name="ID_GRUPPO" minOccurs="1" maxOccurs="1" type="xs:decimal" />
              <xs:element name="NOME_UTENTE" minOccurs="1" maxOccurs="1" type="xs:string" />
              <xs:element name="PASSWORD_UTENTE" minOccurs="1" maxOccurs="1" type="xs:string" />
              <xs:element name="FLG_ABILITATO" nillable="true" minOccurs="1" maxOccurs="1" />
              <xs:element name="DES_UTENTE" type="xs:string" />
              <xs:element name="ACRONIMO" type="xs:string" />
              <xs:element name="FLG_SUPERVISOR" nillable="true" minOccurs="0" maxOccurs="1"
   type="xs:decimal" />

i have set nillable="true" but if my FLG_SUPERVISORE value in xml is blank (null) gives me this error:
how can i change my xsd to enable blach/null value?
(nillable ="true" doesn't work in no other element case)
thanks a lot