diff --git a/src/librustc/mir/interpret/allocation.rs b/src/librustc/mir/interpret/allocation.rs
index bfbfffeb3b8c..d3f87f16313d 100644
--- a/src/librustc/mir/interpret/allocation.rs
+++ b/src/librustc/mir/interpret/allocation.rs
@@ -143,6 +143,11 @@ impl<Tag, Extra> Allocation<Tag, Extra> {
     pub fn inspect_with_undef_and_ptr_outside_interpreter(&self, range: Range<usize>) -> &[u8] {
         &self.bytes[range]
     }
+
+    /// View the undef mask.
+    pub fn undef_mask(&self) -> &UndefMask {
+        &self.undef_mask
+    }
 }
 
 impl<'tcx> rustc_serialize::UseSpecializedDecodable for &'tcx Allocation {}
diff --git a/src/librustc_mir/interpret/memory.rs b/src/librustc_mir/interpret/memory.rs
index 15180265ad29..f7576a41e0f0 100644
--- a/src/librustc_mir/interpret/memory.rs
+++ b/src/librustc_mir/interpret/memory.rs
@@ -653,7 +653,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
                 }
                 relocations.push((i, target_id));
             }
-            if alloc.undef_mask.is_range_defined(i, i + Size::from_bytes(1)).is_ok() {
+            if alloc.undef_mask().is_range_defined(i, i + Size::from_bytes(1)).is_ok() {
                 // this `as usize` is fine, since `i` came from a `usize`
                 let i = i.bytes() as usize;