JAXB fails to generate enum for simple type

Question

I have simpleType Enum which is behaving differently when generating JAXB classes from the below schema:

            <xsd:element name="genderCode" type="GenderCodeType" minOccurs="0" />

            <xsd:simpleType name="GenderCodeType" >
                        <xsd:restriction base="xsd:string">
                            <xsd:enumeration value="F" />
                            <xsd:enumeration value="M" />
                        </xsd:restriction>
                    </xsd:simpleType> 

It generates:

            @XmlElement
            protected String genderCode;

where as this one below generated actual enum type:

            <xsd:element name="genderCode" type="base:Gender" minOccurs="0" />

            <xsd:simpleType name="Gender">
                    <xsd:restriction base="xsd:string">
                        <xsd:enumeration value="M"/>
                        <xsd:enumeration value="F"/>
                        <xsd:enumeration value="U"/>
                    </xsd:restriction>
                </xsd:simpleType>

...

            @XmlType(name = "Gender")
            @XmlEnum
            public enum Gender {
                M, F, U;

                public String value() {
                    return name();
                }

                public static Gender fromValue(String v) {
                    return valueOf(v);
                }
            }

...

    @XmlElement
    protected Gender genderCode;