diff --git a/src/librustc/middle/trans/context.rs b/src/librustc/middle/trans/context.rs
index aca267023c7..56fcbaadbe9 100644
--- a/src/librustc/middle/trans/context.rs
+++ b/src/librustc/middle/trans/context.rs
@@ -27,6 +27,7 @@ use middle::trans::type_::Type;
 
 use util::sha2::Sha256;
 
+use std::cell::RefCell;
 use std::c_str::ToCStr;
 use std::hashmap::{HashMap, HashSet};
 use std::local_data;
@@ -68,7 +69,7 @@ pub struct CrateContext {
      // that is generated
      non_inlineable_statics: HashSet<ast::NodeId>,
      // Cache instances of monomorphized functions
-     monomorphized: HashMap<mono_id, ValueRef>,
+     monomorphized: RefCell<HashMap<mono_id, ValueRef>>,
      monomorphizing: HashMap<ast::DefId, uint>,
      // Cache generated vtables
      vtables: HashMap<(ty::t, mono_id), ValueRef>,
@@ -199,7 +200,7 @@ impl CrateContext {
                   external: HashMap::new(),
                   external_srcs: HashMap::new(),
                   non_inlineable_statics: HashSet::new(),
-                  monomorphized: HashMap::new(),
+                  monomorphized: RefCell::new(HashMap::new()),
                   monomorphizing: HashMap::new(),
                   vtables: HashMap::new(),
                   const_cstr_cache: HashMap::new(),
diff --git a/src/librustc/middle/trans/monomorphize.rs b/src/librustc/middle/trans/monomorphize.rs
index 3350a52e2bc..229a98055c8 100644
--- a/src/librustc/middle/trans/monomorphize.rs
+++ b/src/librustc/middle/trans/monomorphize.rs
@@ -76,13 +76,16 @@ pub fn monomorphic_fn(ccx: @mut CrateContext,
            psubsts.repr(ccx.tcx),
            hash_id);
 
-    match ccx.monomorphized.find(&hash_id) {
-      Some(&val) => {
-        debug!("leaving monomorphic fn {}",
-               ty::item_path_str(ccx.tcx, fn_id));
-        return (val, must_cast);
-      }
-      None => ()
+    {
+        let monomorphized = ccx.monomorphized.borrow();
+        match monomorphized.get().find(&hash_id) {
+          Some(&val) => {
+            debug!("leaving monomorphic fn {}",
+                   ty::item_path_str(ccx.tcx, fn_id));
+            return (val, must_cast);
+          }
+          None => ()
+        }
     }
 
     let tpt = ty::lookup_item_type(ccx.tcx, fn_id);
@@ -201,7 +204,8 @@ pub fn monomorphic_fn(ccx: @mut CrateContext,
 
     let mk_lldecl = || {
         let lldecl = decl_internal_rust_fn(ccx, f.sig.inputs, f.sig.output, s);
-        ccx.monomorphized.insert(hash_id, lldecl);
+        let mut monomorphized = ccx.monomorphized.borrow_mut();
+        monomorphized.get().insert(hash_id, lldecl);
         lldecl
     };