why MultiLineString not undefined

I write a shcema with GML 3.0 and import feature.xsd and xlinks.xsd
but in IntrsrateType complexType I use gml:MultiLineString but failed.
It said "This schema doesnot appear to valid by itself(as a part of another
schema,it might still be OK):
DTD/schema error -element"gml:MultiLineString" is undefined.
I do not understand "gml:Polygon" and "gml:MultiLineString" are both in "geometry.xsd",why gml:MultiLineString is undefined?
who can help me? thanks!
The following is my shcema: