diff --git a/src/contract/mod.rs b/src/contract/mod.rs
index 6f58a14..823d74f 100644
--- a/src/contract/mod.rs
+++ b/src/contract/mod.rs
@@ -118,11 +118,18 @@ impl ContractParameters {
 
         for (outcome, payout_map) in self.outcome_payouts.iter() {
             // Check for unknown outcomes.
-            if let &Outcome::Attestation(outcome_index) = outcome {
-                if outcome_index >= self.event.outcome_messages.len() {
-                    return Err(Error);
+            match outcome {
+                &Outcome::Attestation(outcome_index) => {
+                    if outcome_index >= self.event.outcome_messages.len() {
+                        return Err(Error);
+                    }
                 }
-            }
+                Outcome::Expiry => {
+                    if self.event.expiry.is_none() {
+                        return Err(Error);
+                    }
+                }
+            };
 
             // Check for empty payout map.
             if payout_map.len() == 0 {
diff --git a/src/contract/outcome.rs b/src/contract/outcome.rs
index dd91326..bd83bfb 100644
--- a/src/contract/outcome.rs
+++ b/src/contract/outcome.rs
@@ -80,7 +80,7 @@ pub(crate) fn build_outcome_txs(
             };
 
             let lock_time = match outcome {
-                Outcome::Expiry => LockTime::from_consensus(params.event.expiry),
+                Outcome::Expiry => LockTime::from_consensus(params.event.expiry.ok_or(Error)?),
                 Outcome::Attestation(_) => LockTime::ZERO, // Normal outcome transaction
             };
 
@@ -91,9 +91,9 @@ pub(crate) fn build_outcome_txs(
                 output: vec![outcome_output],
             };
 
-            (outcome, outcome_tx)
+            Ok((outcome, outcome_tx))
         })
-        .collect();
+        .collect::<Result<_, Error>>()?;
 
     let funding_spend_info =
         FundingSpendInfo::new(&params.market_maker, &params.players, params.funding_value)?;
diff --git a/src/oracles.rs b/src/oracles.rs
index 3dfab8f..e0efb72 100644
--- a/src/oracles.rs
+++ b/src/oracles.rs
@@ -1,7 +1,7 @@
 use secp::{MaybePoint, MaybeScalar, Point, Scalar};
 use serde::{Deserialize, Serialize};
 
-use crate::{serialization, OutcomeIndex};
+use crate::{serialization, Outcome, OutcomeIndex};
 
 /// An oracle's announcement of a future event.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
@@ -17,7 +17,8 @@ pub struct EventAnnouncement {
     pub outcome_messages: Vec<Vec<u8>>,
 
     /// The unix timestamp beyond which the oracle is considered to have gone AWOL.
-    pub expiry: u32,
+    /// If set to `None`, the event has no expected expiry.
+    pub expiry: Option<u32>,
 }
 
 impl EventAnnouncement {
@@ -63,4 +64,11 @@ impl EventAnnouncement {
         );
         Some(k + e * d)
     }
+
+    /// Returns an iterator over all possible outcomes in the event.
+    pub fn all_outcomes(&self) -> impl IntoIterator<Item = Outcome> {
+        (0..self.outcome_messages.len())
+            .map(|i| Outcome::Attestation(i))
+            .chain(self.expiry.map(|_| Outcome::Expiry))
+    }
 }