diff --git a/src/lvalue.rs b/src/lvalue.rs
index 9660b8f4eec8..cad4ca9d0290 100644
--- a/src/lvalue.rs
+++ b/src/lvalue.rs
@@ -104,6 +104,15 @@ impl<'tcx> Global<'tcx> {
             initialized: false,
         }
     }
+
+    pub(super) fn initialized(ty: Ty<'tcx>, value: Value, mutable: bool) -> Self {
+        Global {
+            value,
+            mutable,
+            ty,
+            initialized: true,
+        }
+    }
 }
 
 impl<'a, 'tcx> EvalContext<'a, 'tcx> {
diff --git a/src/step.rs b/src/step.rs
index 27eb26e333ac..110a7b2252a8 100644
--- a/src/step.rs
+++ b/src/step.rs
@@ -13,6 +13,7 @@ use error::{EvalResult, EvalError};
 use eval_context::{EvalContext, StackPopCleanup};
 use lvalue::{Global, GlobalId, Lvalue};
 use value::{Value, PrimVal};
+use memory::Pointer;
 use syntax::codemap::Span;
 
 impl<'a, 'tcx> EvalContext<'a, 'tcx> {
@@ -158,6 +159,11 @@ impl<'a, 'b, 'tcx> ConstantExtractor<'a, 'b, 'tcx> {
         if self.ecx.globals.contains_key(&cid) {
             return;
         }
+        if self.ecx.tcx.has_attr(def_id, "linkage") {
+            trace!("Initializing an extern global with NULL");
+            self.ecx.globals.insert(cid, Global::initialized(self.ecx.tcx.type_of(def_id), Value::ByVal(PrimVal::Ptr(Pointer::from_int(0))), !shared));
+            return;
+        }
         self.try(|this| {
             let mir = this.ecx.load_mir(instance.def)?;
             this.ecx.globals.insert(cid, Global::uninitialized(mir.return_ty));
@@ -178,6 +184,7 @@ impl<'a, 'b, 'tcx> ConstantExtractor<'a, 'b, 'tcx> {
             )
         });
     }
+
     fn try<F: FnOnce(&mut Self) -> EvalResult<'tcx>>(&mut self, f: F) {
         if let Ok(ref mut n) = *self.new_constants {
             *n += 1;