diff --git a/lib/src/async/concat.dart b/lib/src/async/concat.dart
index cd02a803..f894cf97 100644
--- a/lib/src/async/concat.dart
+++ b/lib/src/async/concat.dart
@@ -71,7 +71,10 @@ class _ConcatStream<T> extends Stream<T> {
     }, onResume: () {
       if (currentSubscription != null) currentSubscription.resume();
     }, onCancel: () {
-      if (currentSubscription != null) return currentSubscription.cancel();
+      if (currentSubscription != null)
+        return currentSubscription.cancel();
+      else
+        return null;
     });
 
     nextStream();