diff --git a/lib/VMCore/Dominators.cpp b/lib/VMCore/Dominators.cpp
index a0e818fc313..7019c98d89e 100644
--- a/lib/VMCore/Dominators.cpp
+++ b/lib/VMCore/Dominators.cpp
@@ -19,6 +19,7 @@
 #include "llvm/Assembly/Writer.h"
 #include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/SetOperations.h"
+#include "llvm/Instructions.h"
 #include <algorithm>
 using namespace llvm;
 
@@ -265,6 +266,11 @@ bool DominatorSetBase::dominates(Instruction *A, Instruction *B) const {
   BasicBlock *BBA = A->getParent(), *BBB = B->getParent();
   if (BBA != BBB) return dominates(BBA, BBB);
 
+  // It is not possible to determie dominance between two PHI nodes 
+  // based on their ordering.
+  if (isa<PHINode>(A) && isa<PHINode>(B)) 
+    return false;
+
   // Loop through the basic block until we find A or B.
   BasicBlock::iterator I = BBA->begin();
   for (; &*I != A && &*I != B; ++I) /*empty*/;