diff --git a/dist/Deferred.js b/dist/Deferred.js
index dd5334d6e42b0881f411fca56688e639cd49c176..ea200a80d6186f07a29d0d231ff1f038433bfec3 100644
--- a/dist/Deferred.js
+++ b/dist/Deferred.js
@@ -81,8 +81,12 @@ const _await = self => internalEffect.callback(resume => {
   self.resumes ??= [];
   self.resumes.push(resume);
   return internalEffect.sync(() => {
+    // Completion resumes every waiter and clears the array, so a cleanup
+    // that runs after completion (an interrupt racing the resume) has
+    // nothing to unregister.
+    if (!self.resumes) return;
     const index = self.resumes.indexOf(resume);
-    self.resumes.splice(index, 1);
+    if (index >= 0) self.resumes.splice(index, 1);
   });
 });
 export {
diff --git a/src/Deferred.ts b/src/Deferred.ts
index f6d37948bfbe690a7998b06c562e434e1b8ae084..da91e4dc0fd32e534fbc72f7beb0514e67e4ce74 100644
--- a/src/Deferred.ts
+++ b/src/Deferred.ts
@@ -188,8 +188,12 @@ const _await = <A, E>(self: Deferred<A, E>): Effect<A, E> =>
     self.resumes ??= []
     self.resumes.push(resume)
     return internalEffect.sync(() => {
-      const index = self.resumes!.indexOf(resume)
-      self.resumes!.splice(index, 1)
+      // Completion resumes every waiter and clears the array, so a cleanup
+      // that runs after completion (an interrupt racing the resume) has
+      // nothing to unregister.
+      if (!self.resumes) return
+      const index = self.resumes.indexOf(resume)
+      if (index >= 0) self.resumes.splice(index, 1)
     })
   })