diff --git a/lib/ubsan/ubsan_type_hash_itanium.cc b/lib/ubsan/ubsan_type_hash_itanium.cc
index 729c4a0..dcce0dd 100644
--- a/lib/ubsan/ubsan_type_hash_itanium.cc
+++ b/lib/ubsan/ubsan_type_hash_itanium.cc
@@ -197,9 +197,9 @@
 };
 VtablePrefix *getVtablePrefix(void *Vtable) {
   VtablePrefix *Vptr = reinterpret_cast<VtablePrefix*>(Vtable);
-  if (!IsAccessibleMemoryRange((uptr)Vptr, sizeof(VtablePrefix)))
-    return nullptr;
   VtablePrefix *Prefix = Vptr - 1;
+  if (!IsAccessibleMemoryRange((uptr)Prefix, sizeof(VtablePrefix)))
+    return nullptr;
   if (!Prefix->TypeInfo)
     // This can't possibly be a valid vtable.
     return nullptr;