diff --git a/llvm/unittests/Support/AllocatorTest.cpp b/llvm/unittests/Support/AllocatorTest.cpp
index 665d1b7e38f0..b361b0aa7252 100644
--- a/llvm/unittests/Support/AllocatorTest.cpp
+++ b/llvm/unittests/Support/AllocatorTest.cpp
@@ -105,6 +105,7 @@ TEST(AllocatorTest, TestAlignment) {
 // we end up creating a slab for it.
 TEST(AllocatorTest, TestZero) {
   BumpPtrAllocator Alloc;
+  Alloc.setRedZoneSize(0); // else our arithmetic is all off
   EXPECT_EQ(0u, Alloc.GetNumSlabs());
   EXPECT_EQ(0u, Alloc.getBytesAllocated());