diff a/src/hotspot/share/gc/shenandoah/shenandoahObjArrayAllocator.cpp b/src/hotspot/share/gc/shenandoah/shenandoahObjArrayAllocator.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahObjArrayAllocator.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahObjArrayAllocator.cpp
@@ -69,11 +69,14 @@
   region->record_pin();
 
   // Always initialize the mem with primitive array first so GC won't look into the elements in the array.
   // For obj array, the header will be corrected to object array after clearing the memory.
   Klass* filling_klass = _klass;
-  const bool is_ref_type = is_reference_type(element_type);
+
+  // Flat arrays containing oops are not supported and only contain primitives
+  // from here on out.
+  const bool is_ref_type = (element_type != T_FLAT_ELEMENT) && is_reference_type(element_type);
 
   if (is_ref_type) {
     filling_klass = LP64_ONLY(UseCompressedOops ? Universe::intArrayKlass() : Universe::longArrayKlass()) NOT_LP64(Universe::intArrayKlass());
     assert(type2aelembytes(ArrayKlass::cast(filling_klass)->element_type()) == type2aelembytes(element_type), "filling element size must match ref size");
   }