diff --git a/src/future/future_group.rs b/src/future/future_group.rs
index a64a418..df18a0a 100644
--- a/src/future/future_group.rs
+++ b/src/future/future_group.rs
@@ -67,6 +67,7 @@ pub struct FutureGroup<F> {
     wakers: WakerVec,
     states: PollVec,
     keys: BTreeSet<usize>,
+    capacity: usize,
 }
 
 impl<T: Debug> Debug for FutureGroup<T> {
@@ -108,6 +109,7 @@ impl<F> FutureGroup<F> {
             wakers: WakerVec::new(capacity),
             states: PollVec::new(capacity),
             keys: BTreeSet::new(),
+            capacity,
         }
     }
 
@@ -142,7 +144,7 @@ impl<F> FutureGroup<F> {
     /// # let group: FutureGroup<usize> = group;
     /// ```
     pub fn capacity(&self) -> usize {
-        self.futures.capacity()
+        self.capacity
     }
 
     /// Returns true if there are no futures currently active in the group.
@@ -232,10 +234,11 @@ impl<F: Future> FutureGroup<F> {
         // If our slab allocated more space we need to
         // update our tracking structures along with it.
         let capacity = self.capacity();
-        let max_len = capacity.max(index);
+        let max_len = capacity.max(index + 1);
         if max_len > capacity {
             self.wakers.resize(max_len);
             self.states.resize(max_len);
+            self.capacity = max_len;
         }
 
         // Set the corresponding state
@@ -424,4 +427,14 @@ mod test {
             assert!(group.is_empty());
         });
     }
+
+    #[test]
+    fn insert_many() {
+        futures_lite::future::block_on(async {
+            let mut group = FutureGroup::new();
+            for _ in 0..100 {
+                group.insert(future::ready(2));
+            }
+        });
+    }
 }
diff --git a/src/stream/stream_group.rs b/src/stream/stream_group.rs
index 02ffe42..e5d8f0d 100644
--- a/src/stream/stream_group.rs
+++ b/src/stream/stream_group.rs
@@ -66,6 +66,7 @@ pub struct StreamGroup<S> {
     states: PollVec,
     keys: BTreeSet<usize>,
     key_removal_queue: SmallVec<[usize; 10]>,
+    capacity: usize,
 }
 
 impl<T: Debug> Debug for StreamGroup<T> {
@@ -108,6 +109,7 @@ impl<S> StreamGroup<S> {
             states: PollVec::new(capacity),
             keys: BTreeSet::new(),
             key_removal_queue: smallvec![],
+            capacity,
         }
     }
 
@@ -141,7 +143,7 @@ impl<S> StreamGroup<S> {
     /// # let group: StreamGroup<usize> = group;
     /// ```
     pub fn capacity(&self) -> usize {
-        self.streams.capacity()
+        self.capacity
     }
 
     /// Returns true if there are no futures currently active in the group.
@@ -224,17 +226,19 @@ impl<S: Stream> StreamGroup<S> {
     where
         S: Stream,
     {
+        dbg!();
         let index = self.streams.insert(stream);
-        self.keys.insert(index);
+        self.keys.insert(dbg!(index));
         let key = Key(index);
 
         // If our slab allocated more space we need to
         // update our tracking structures along with it.
         let capacity = self.capacity();
-        let max_len = capacity.max(index);
+        let max_len = capacity.max(index + 1);
         if max_len > capacity {
             self.wakers.resize(max_len);
             self.states.resize(max_len);
+            self.capacity = max_len;
         }
 
         // Set the corresponding state
@@ -443,4 +447,14 @@ mod test {
             assert!(group.is_empty());
         });
     }
+
+    #[test]
+    fn insert_many() {
+        futures_lite::future::block_on(async {
+            let mut group = StreamGroup::new();
+            for _ in 0..100 {
+                group.insert(stream::once(2));
+            }
+        });
+    }
 }