diff --git a/include/llvm/ADT/IntervalMap.h b/include/llvm/ADT/IntervalMap.h
index b128f31a8d2..dd4eec3c63c 100644
--- a/include/llvm/ADT/IntervalMap.h
+++ b/include/llvm/ADT/IntervalMap.h
@@ -1235,7 +1235,7 @@ public:
     if (!branched()) {
       assert(rootOffset && "Cannot decrement begin()");
       --rootOffset;
-    } else if (treeLeafOffset())
+    } else if (valid() && treeLeafOffset())
       --treeLeafOffset();
     else
       treeDecrement();
diff --git a/unittests/ADT/IntervalMapTest.cpp b/unittests/ADT/IntervalMapTest.cpp
index 5c8b61f278d..c7def84340a 100644
--- a/unittests/ADT/IntervalMapTest.cpp
+++ b/unittests/ADT/IntervalMapTest.cpp
@@ -352,6 +352,16 @@ TEST(IntervalMapTest, Branched) {
   EXPECT_FALSE(I.valid());
   EXPECT_TRUE(I == map.end());
 
+  // Backwards iteration.
+  for (unsigned i = 99; i; --i) {
+    --I;
+    ASSERT_TRUE(I.valid());
+    EXPECT_EQ(10*i, I.start());
+    EXPECT_EQ(10*i+5, I.stop());
+    EXPECT_EQ(i, *I);
+  }
+  EXPECT_TRUE(I == map.begin());
+
 }
 
 } // namespace