Help me! GenerationAMesh

Hi, please help me. The error is :

mSubMesh.vertexData = new VertexData();
"Assertion failed!
Expression: ms_Singleton"

MOGRE 1.4.8 and VS2008

public MeshBuilderHelper(String name, String resourcegroup,
bool usesharedvertices, uint vertexstart, uint vertexcount)
{
mName = name;
mResourceGroup = resourcegroup;
mVertextStart = vertexstart;
mVertexCount = vertexcount;

// Return now if already exists
if (MeshManager.Singleton.ResourceExists(name))
return;

mMeshPtr = MeshManager.Singleton.CreateManual(mName, mResourceGroup);
mSubMesh = mMeshPtr.CreateSubMesh();
mSubMesh.useSharedVertices = usesharedvertices;

mSubMesh.vertexData = new VertexData();

mSubMesh.vertexData.vertexStart = mVertextStart;
mSubMesh.vertexData.vertexCount = mVertexCount;
offset = 0;
mIndexType = HardwareIndexBuffer.IndexType.IT_16BIT;
}