diff --git a/lib/Support/ScaledNumber.cpp b/lib/Support/ScaledNumber.cpp
index 3fe027ba331..fc6d4e7be43 100644
--- a/lib/Support/ScaledNumber.cpp
+++ b/lib/Support/ScaledNumber.cpp
@@ -220,6 +220,9 @@ std::string ScaledNumberBase::toString(uint64_t D, int16_t E, int Width,
   } else if (E > -64) {
     Above0 = D >> -E;
     Below0 = D << (64 + E);
+  } else if (E == -64) {
+    // Special case: shift by 64 bits is undefined behavior.
+    Below0 = D;
   } else if (E > -120) {
     Below0 = D >> (-E - 64);
     Extra = D << (128 + E);