EnvDTE::Document.Open always return null

[Zolix2010]

hi,
im trying to open a document from the documents list but it always returns null.
this is my code:
<code>
// Find .dsp file name
m_ActiveProject->get_FullName(&l_ProjectName);

// Open .dsp file as text document
CComPtr<EnvDTE::Documents> l_pDocuments;
m_pDTE->get_Documents(&l_pDocuments);

CComPtr<EnvDTE::Document> l_pDocument;
l_pDocuments->Open(l_ProjectName,L"Text",false,&l_pDocument);
</code>

am i doing something wrong?