diff --git a/lib/Support/raw_ostream.cpp b/lib/Support/raw_ostream.cpp
index c9cf249500d..4927e9a7b9d 100644
--- a/lib/Support/raw_ostream.cpp
+++ b/lib/Support/raw_ostream.cpp
@@ -121,7 +121,8 @@ raw_ostream &raw_ostream::operator<<(unsigned long N) {
 raw_ostream &raw_ostream::operator<<(long N) {
   if (N <  0) {
     *this << '-';
-    N = -N;
+    // Avoid undefined behavior on LONG_MIN with a cast.
+    N = -(unsigned long)N;
   }
 
   return this->operator<<(static_cast<unsigned long>(N));