diff --git a/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index 58d5801ea084..76da97068298 100644
--- a/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -1774,7 +1774,7 @@ struct VarArgAMD64Helper : public VarArgHelper {
     // Unpoison the whole __va_list_tag.
     // FIXME: magic ABI constants.
     IRB.CreateMemSet(ShadowPtr, Constant::getNullValue(IRB.getInt8Ty()),
-                     /* size */24, /* alignment */16, false);
+                     /* size */24, /* alignment */8, false);
   }
 
   void visitVACopyInst(VACopyInst &I) {
@@ -1785,7 +1785,7 @@ struct VarArgAMD64Helper : public VarArgHelper {
     // Unpoison the whole __va_list_tag.
     // FIXME: magic ABI constants.
     IRB.CreateMemSet(ShadowPtr, Constant::getNullValue(IRB.getInt8Ty()),
-                     /* size */ 24, /* alignment */ 16, false);
+                     /* size */24, /* alignment */8, false);
   }
 
   void finalizeInstrumentation() {
diff --git a/test/Instrumentation/MemorySanitizer/msan_basic.ll b/test/Instrumentation/MemorySanitizer/msan_basic.ll
index 20957fb36f4e..cd90f8836aa0 100644
--- a/test/Instrumentation/MemorySanitizer/msan_basic.ll
+++ b/test/Instrumentation/MemorySanitizer/msan_basic.ll
@@ -534,3 +534,16 @@ define <8 x i8*> @VectorOfPointers(<8 x i8*>* %p) nounwind uwtable {
 ; CHECK: load <8 x i8*>*
 ; CHECK: store <8 x i64> {{.*}} @__msan_retval_tls
 ; CHECK: ret <8 x i8*>
+
+; Test handling of va_copy.
+
+declare void @llvm.va_copy(i8*, i8*) nounwind
+
+define void @VACopy(i8* %p1, i8* %p2) nounwind uwtable {
+  call void @llvm.va_copy(i8* %p1, i8* %p2) nounwind
+  ret void
+}
+
+; CHECK: @VACopy
+; CHECK: call void @llvm.memset.p0i8.i64({{.*}}, i8 0, i64 24, i32 8, i1 false)
+; CHECK: ret void