diff --git a/src/interpreter.rs b/src/interpreter.rs
index 8db2f18f46fa..f1ec0c65e8a2 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -6,7 +6,7 @@ use std::error::Error;
 use std::fmt;
 
 use memory::{FieldRepr, Memory, Pointer, Repr};
-use primval;
+use primval::{self, PrimVal};
 
 const TRACE_EXECUTION: bool = true;
 
@@ -277,7 +277,10 @@ impl<'a, 'tcx: 'a> Interpreter<'a, 'tcx> {
 
                         ty::AdtKind::Enum => match dest_repr {
                             Repr::Sum { ref discr, ref variants, .. } => {
-                                // TODO(tsion): Write the discriminant value.
+                                if discr.size() > 0 {
+                                    let discr_val = PrimVal::from_int(variant_idx as i64, discr);
+                                    try!(self.memory.write_primval(dest, discr_val));
+                                }
                                 self.assign_to_product(
                                     dest.offset(discr.size()),
                                     &variants[variant_idx],
diff --git a/src/primval.rs b/src/primval.rs
index 9442adbaf7e7..3fbcd6636404 100644
--- a/src/primval.rs
+++ b/src/primval.rs
@@ -1,5 +1,7 @@
 use rustc::mir::repr as mir;
 
+use memory::Repr;
+
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum PrimVal {
     Bool(bool),
@@ -9,6 +11,19 @@ pub enum PrimVal {
     I64(i64),
 }
 
+impl PrimVal {
+    pub fn from_int(n: i64, repr: &Repr) -> Self {
+        // TODO(tsion): Use checked casts.
+        match *repr {
+            Repr::I8 => PrimVal::I8(n as i8),
+            Repr::I16 => PrimVal::I16(n as i16),
+            Repr::I32 => PrimVal::I32(n as i32),
+            Repr::I64 => PrimVal::I64(n),
+            _ => panic!("attempted to make integer primval from non-integer repr"),
+        }
+    }
+}
+
 pub fn binary_op(bin_op: mir::BinOp, left: PrimVal, right: PrimVal) -> PrimVal {
     macro_rules! int_binops {
         ($v:ident, $l:ident, $r:ident) => ({
diff --git a/test/sums.rs b/test/sums.rs
index e6545e81c00b..d04f0b2e7cf2 100644
--- a/test/sums.rs
+++ b/test/sums.rs
@@ -1,6 +1,25 @@
 #![feature(custom_attribute)]
 #![allow(dead_code, unused_attributes)]
 
+enum Unit { Unit }
+
+#[miri_run]
+fn return_unit() -> Unit {
+    Unit::Unit
+}
+
+enum MyBool { False, True }
+
+#[miri_run]
+fn return_true() -> MyBool {
+    MyBool::True
+}
+
+#[miri_run]
+fn return_false() -> MyBool {
+    MyBool::False
+}
+
 #[miri_run]
 fn return_none() -> Option<i64> {
     None