Memory Allocation Problem

I'm trying to allocate memory on lsass.exe process but it always fail.

Other tools i try like sam password dump works.

What could be the problem?

Example code i use to allocate memory on that process.

// Enable the debug privilege
if( SetDebugPrivilege() != 0 )

// Get the LSASS pid
dwPid = GetLsassPid();

// Open lsass
hLsassProc = OpenProcess( PROCESS_ALL_ACCESS, FALSE, dwPid );.

// Allocate memory in remote proc
pRemoteAlloc = VirtualAllocEx( hLsassProc, NULL, 1000, MEM_COMMIT, PAGE_READWRITE );

Result:

pRemoteAlloc = NULL

GetLastError = 5

....

I'm using win 7 x86 sp1 and vsc++ 2010