diff --git a/src/share/vm/prims/unsafe.cpp b/src/share/vm/prims/unsafe.cpp
index f76b2474e4602a2031d5ba13247bbda15e74c870..b96f635898a373525f5b99042c4a9a2ca9ddef48 100644
--- a/src/share/vm/prims/unsafe.cpp
+++ b/src/share/vm/prims/unsafe.cpp
@@ -110,6 +110,8 @@ inline jint invocation_key_to_method_slot(jint key) {
 
 inline void* index_oop_from_field_offset_long(oop p, jlong field_offset) {
   jlong byte_offset = field_offset_to_byte_offset(field_offset);
+  // Don't allow unsafe to be used to read or write the header word of oops
+  assert(p == NULL || field_offset >= oopDesc::header_size(), "offset must be outside of header");
 #ifdef ASSERT
   if (p != NULL) {
     assert(byte_offset >= 0 && byte_offset <= (jlong)MAX_OBJECT_SIZE, "sane offset");