diff --git a/src/future/future_group.rs b/src/future/future_group.rs
index 82fd60e..dd775e0 100644
--- a/src/future/future_group.rs
+++ b/src/future/future_group.rs
@@ -270,7 +270,7 @@ impl<F: Future> FutureGroup<F> {
 
         // Set the corresponding state
         this.states[index].set_pending();
-        let mut readiness = this.wakers.readiness().lock().unwrap();
+        let mut readiness = this.wakers.readiness();
         readiness.set_ready(index);
 
         key
diff --git a/src/lib.rs b/src/lib.rs
index 0d3bf39..c3ed456 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -82,10 +82,13 @@ pub mod prelude {
     pub use super::stream::Merge as _;
     pub use super::stream::Zip as _;
 
+    #[cfg(feature = "alloc")]
     pub use super::concurrent_stream::ConcurrentStream;
 }
 
+#[cfg(feature = "alloc")]
 pub mod concurrent_stream;
+
 pub mod future;
 pub mod stream;
 
diff --git a/src/stream/stream_ext.rs b/src/stream/stream_ext.rs
index c9e44df..b025015 100644
--- a/src/stream/stream_ext.rs
+++ b/src/stream/stream_ext.rs
@@ -27,6 +27,7 @@ pub trait StreamExt: Stream {
         S2: IntoStream<Item = T>;
 
     /// Convert into a concurrent stream.
+    #[cfg(feature = "alloc")]
     fn co(self) -> FromStream<Self>
     where
         Self: Sized,