A question about PostDominatorTree::findNearestCommonDominator()

Hi There

I got a question about the PostDominatorTree::findNearestCommonDominator(), which
I used to find the the postdominator of two branches of a conditional instruction.

I encoded the following code into KLEE

     llvm::PostDominatorTree *pDominator = (llvm::PostDominatorTree*)llvm::createPostDomTree();
     BasicBlock *cpdBB = pDominator->findNearestCommonDominator(trueBB, falseBB);
     if (cpdBB) {
       std::cout << "The nearest common post-dominator for branch inst: " << std::endl;

Then I wrote a very simple test case to test it, the CFG is shown as follows:
              / \
    trueBB falseBB
              \ /

I expect to see the postdominator of trueBB and falseBB as mergeBB, but the function always
returns "falseBB" as post-dominator, did I do anything wrong during calling this function? My
llvm version is 2.8-release

Any hint is highly appreciated.