diff --git a/src/comp/middle/gc.rs b/src/comp/middle/gc.rs
index 4501f96fed67..878762294c62 100644
--- a/src/comp/middle/gc.rs
+++ b/src/comp/middle/gc.rs
@@ -3,6 +3,7 @@
 import lib::llvm::False;
 import lib::llvm::True;
 import lib::llvm::llvm::ValueRef;
+import middle::trans;
 import middle::trans::get_tydesc;
 import middle::trans_common::*;
 import middle::ty;
@@ -38,6 +39,10 @@ fn add_gc_root(cx: &@block_ctxt, llval: ValueRef, ty: ty::t) -> @block_ctxt {
 
     let gc_cx = bcx_ccx(cx).gc_cx;
 
+    // FIXME (issue #839): For now, we are unconditionally zeroing out all
+    // GC-relevant types. Eventually we should use typestate for this.
+    bcx = trans::zero_alloca(bcx, llval, ty).bcx;
+
     let ti = none;
     let td_r = get_tydesc(bcx, ty, false, ti);
     bcx = td_r.result.bcx;