diff --git a/m4/curl-compilers.m4 b/m4/curl-compilers.m4
index 5890324468..673f640979 100644
--- a/m4/curl-compilers.m4
+++ b/m4/curl-compilers.m4
@@ -881,6 +881,11 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
           if test "$compiler_num" -ge "101"; then
             tmp_CFLAGS="$tmp_CFLAGS -Wunused"
           fi
+          #
+          dnl Only clang 2.9 or later
+          if test "$compiler_num" -ge "209"; then
+            tmp_CFLAGS="$tmp_CFLAGS -Wshift-sign-overflow"
+          fi
         fi
         ;;
         #