diff --git a/lib/Analysis/RegionInfo.cpp b/lib/Analysis/RegionInfo.cpp
index 81ef6505e61..857702570f2 100644
--- a/lib/Analysis/RegionInfo.cpp
+++ b/lib/Analysis/RegionInfo.cpp
@@ -114,7 +114,8 @@ void Region::replaceExitRecursive(BasicBlock *NewExit) {
 bool Region::contains(const BasicBlock *B) const {
   BasicBlock *BB = const_cast<BasicBlock*>(B);
 
-  assert(DT->getNode(BB) && "BB not part of the dominance tree");
+  if (!DT->getNode(BB))
+    return false;
 
   BasicBlock *entry = getEntry(), *exit = getExit();
 
diff --git a/test/Analysis/RegionInfo/unreachable_bb.ll b/test/Analysis/RegionInfo/unreachable_bb.ll
new file mode 100644
index 00000000000..626ccbe304c
--- /dev/null
+++ b/test/Analysis/RegionInfo/unreachable_bb.ll
@@ -0,0 +1,29 @@
+; RUN: opt -regions -analyze < %s | FileCheck %s
+
+; We should not crash if there are some bbs that are not reachable.
+define void @f() {
+entry:
+  br label %for.pre
+
+notintree:                                        ; No predecessors!
+  br label %ret
+
+for.pre:                                          ; preds = %entry
+  br label %for
+
+for:                                              ; preds = %for.inc, %for.pre
+  %indvar = phi i64 [ 0, %for.pre ], [ %indvar.next, %for.inc ]
+  %exitcond = icmp ne i64 %indvar, 200
+  br i1 %exitcond, label %for.inc, label %ret
+
+for.inc:                                          ; preds = %for
+  %indvar.next = add i64 %indvar, 1
+  br label %for
+
+ret:                                              ; preds = %for, %notintree
+  ret void
+}
+
+; CHECK: [0] entry => <Function Return>
+; CHECK:   [1] for => ret
+