diff --git a/lib/Support/KnownBits.cpp b/lib/Support/KnownBits.cpp
index 0f36c6a9ef1..a46a90bb97d 100644
--- a/lib/Support/KnownBits.cpp
+++ b/lib/Support/KnownBits.cpp
@@ -85,7 +85,11 @@ KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
 
 KnownBits KnownBits::sextInReg(unsigned SrcBitWidth) const {
   unsigned BitWidth = getBitWidth();
-  assert(BitWidth >= SrcBitWidth && "Illegal sext-in-register");
+  assert(0 < SrcBitWidth && SrcBitWidth <= BitWidth &&
+         "Illegal sext-in-register");
+
+  if (SrcBitWidth == BitWidth)
+    return *this;
 
   // Sign extension.  Compute the demanded bits in the result that are not
   // present in the input.
diff --git a/unittests/Support/KnownBitsTest.cpp b/unittests/Support/KnownBitsTest.cpp
index 991096098b8..4e69df49837 100644
--- a/unittests/Support/KnownBitsTest.cpp
+++ b/unittests/Support/KnownBitsTest.cpp
@@ -427,7 +427,7 @@ TEST(KnownBitsTest, SExtOrTrunc) {
 
 TEST(KnownBitsTest, SExtInReg) {
   unsigned Bits = 4;
-  for (unsigned FromBits = 1; FromBits != Bits; ++FromBits) {
+  for (unsigned FromBits = 1; FromBits <= Bits; ++FromBits) {
     ForeachKnownBits(Bits, [&](const KnownBits &Known) {
       APInt CommonOne = APInt::getAllOnesValue(Bits);
       APInt CommonZero = APInt::getAllOnesValue(Bits);