diff --git a/llvm/include/llvm/ADT/SmallBitVector.h b/llvm/include/llvm/ADT/SmallBitVector.h
--- a/llvm/include/llvm/ADT/SmallBitVector.h
+++ b/llvm/include/llvm/ADT/SmallBitVector.h
@@ -287,11 +287,11 @@
   /// Returns -1 if the next unset bit is not found.
   int find_next_unset(unsigned Prev) const {
     if (isSmall()) {
-      ++Prev;
       uintptr_t Bits = getSmallBits();
       // Mask in previous bits.
-      uintptr_t Mask = (uintptr_t(1) << Prev) - 1;
-      Bits |= Mask;
+      Bits |= (uintptr_t(1) << (Prev + 1)) - 1;
+      // Mask in unused bits.
+      Bits |= ~uintptr_t(0) << getSmallSize();
 
       if (Bits == ~uintptr_t(0) || Prev + 1 >= getSmallSize())
         return -1;
diff --git a/llvm/unittests/ADT/BitVectorTest.cpp b/llvm/unittests/ADT/BitVectorTest.cpp
--- a/llvm/unittests/ADT/BitVectorTest.cpp
+++ b/llvm/unittests/ADT/BitVectorTest.cpp
@@ -297,6 +297,18 @@
   EXPECT_EQ(-1, A.find_last_unset());
 
   A.resize(20);
+  ASSERT_TRUE(SmallBitVectorIsSmallMode(A));
+  EXPECT_EQ(-1, A.find_first());
+  EXPECT_EQ(-1, A.find_last());
+  EXPECT_EQ(-1, A.find_next(5));
+  EXPECT_EQ(-1, A.find_next(19));
+  EXPECT_EQ(-1, A.find_prev(5));
+  EXPECT_EQ(-1, A.find_prev(20));
+  EXPECT_EQ(0, A.find_first_unset());
+  EXPECT_EQ(19, A.find_last_unset());
+  EXPECT_EQ(6, A.find_next_unset(5));
+  EXPECT_EQ(-1, A.find_next_unset(19));
+
   A.set(3);
   A.set(4);
   A.set(16);
@@ -319,6 +331,19 @@
   EXPECT_EQ(5, A.find_next_unset(4));
   EXPECT_EQ(13, A.find_next_unset(12));
   EXPECT_EQ(17, A.find_next_unset(15));
+
+  A.set();
+  ASSERT_TRUE(SmallBitVectorIsSmallMode(A));
+  EXPECT_EQ(0, A.find_first());
+  EXPECT_EQ(19, A.find_last());
+  EXPECT_EQ(6, A.find_next(5));
+  EXPECT_EQ(-1, A.find_next(19));
+  EXPECT_EQ(4, A.find_prev(5));
+  EXPECT_EQ(19, A.find_prev(20));
+  EXPECT_EQ(-1, A.find_first_unset());
+  EXPECT_EQ(-1, A.find_last_unset());
+  EXPECT_EQ(-1, A.find_next_unset(5));
+  EXPECT_EQ(-1, A.find_next_unset(19));
 }
 
 TEST(BitVectorTest, FindInRangeMultiWord) {