diff --git a/src/librustc/middle/trans/context.rs b/src/librustc/middle/trans/context.rs
index eaf94568d934..449862c898d1 100644
--- a/src/librustc/middle/trans/context.rs
+++ b/src/librustc/middle/trans/context.rs
@@ -93,7 +93,7 @@ pub struct CrateContext {
 
      module_data: RefCell<HashMap<~str, ValueRef>>,
      lltypes: RefCell<HashMap<ty::t, Type>>,
-     llsizingtypes: HashMap<ty::t, Type>,
+     llsizingtypes: RefCell<HashMap<ty::t, Type>>,
      adt_reprs: HashMap<ty::t, @adt::Repr>,
      symbol_hasher: Sha256,
      type_hashcodes: HashMap<ty::t, @str>,
@@ -204,7 +204,7 @@ impl CrateContext {
                   impl_method_cache: RefCell::new(HashMap::new()),
                   module_data: RefCell::new(HashMap::new()),
                   lltypes: RefCell::new(HashMap::new()),
-                  llsizingtypes: HashMap::new(),
+                  llsizingtypes: RefCell::new(HashMap::new()),
                   adt_reprs: HashMap::new(),
                   symbol_hasher: symbol_hasher,
                   type_hashcodes: HashMap::new(),
diff --git a/src/librustc/middle/trans/type_of.rs b/src/librustc/middle/trans/type_of.rs
index 7f3cee7c4024..2270b5aa0581 100644
--- a/src/librustc/middle/trans/type_of.rs
+++ b/src/librustc/middle/trans/type_of.rs
@@ -101,9 +101,12 @@ pub fn type_of_fn_from_ty(cx: &mut CrateContext, fty: ty::t) -> Type {
 //     recursive types. For example, enum types rely on this behavior.
 
 pub fn sizing_type_of(cx: &mut CrateContext, t: ty::t) -> Type {
-    match cx.llsizingtypes.find_copy(&t) {
-        Some(t) => return t,
-        None => ()
+    {
+        let llsizingtypes = cx.llsizingtypes.borrow();
+        match llsizingtypes.get().find_copy(&t) {
+            Some(t) => return t,
+            None => ()
+        }
     }
 
     let llsizingty = match ty::get(t).sty {
@@ -166,7 +169,8 @@ pub fn sizing_type_of(cx: &mut CrateContext, t: ty::t) -> Type {
         }
     };
 
-    cx.llsizingtypes.insert(t, llsizingty);
+    let mut llsizingtypes = cx.llsizingtypes.borrow_mut();
+    llsizingtypes.get().insert(t, llsizingty);
     llsizingty
 }