diff --git a/public/appframework/IAppSystem.h b/public/appframework/IAppSystem.h
index 20f9285c..c83dcea7 100644
--- a/public/appframework/IAppSystem.h
+++ b/public/appframework/IAppSystem.h
@@ -76,6 +76,9 @@ public:
 
 	// Reconnect to a particular interface
 	virtual void Reconnect( CreateInterfaceFn factory, const char *pInterfaceName ) = 0;
+
+	// New in Dota. Seems to do nothing but return true in most or all cases.
+	virtual bool UnknownFunc1() = 0;
 };
 
 
diff --git a/public/filesystem_passthru.h b/public/filesystem_passthru.h
index 01db1a93..5617a318 100644
--- a/public/filesystem_passthru.h
+++ b/public/filesystem_passthru.h
@@ -95,6 +95,7 @@ public:
 	virtual const AppSystemInfo_t* GetDependencies()															{ return m_pFileSystemPassThru->GetDependencies(); }
 	virtual AppSystemTier_t GetTier()																			{ return m_pFileSystemPassThru->GetTier(); }
 	virtual void Reconnect( CreateInterfaceFn factory, const char *pInterfaceName )								{ m_pFileSystemPassThru->Reconnect( factory, pInterfaceName ); }
+	virtual bool UnknownFunc1()																					{ return m_pFileSystemPassThru->UnknownFunc1(); }
 
 	virtual void			RemoveAllSearchPaths( void )														{ m_pFileSystemPassThru->RemoveAllSearchPaths(); }
 	virtual void			AddSearchPath( const char *pPath, const char *pathID, SearchPathAdd_t addType )		{ m_pFileSystemPassThru->AddSearchPath( pPath, pathID, addType ); }