diff --git a/timely/src/dataflow/operators/capability.rs b/timely/src/dataflow/operators/capability.rs
index 8ba993726..6235572e6 100644
--- a/timely/src/dataflow/operators/capability.rs
+++ b/timely/src/dataflow/operators/capability.rs
@@ -37,19 +37,19 @@ use crate::dataflow::channels::pullers::counter::ConsumedGuard;
 pub trait CapabilityTrait<T: Timestamp> {
     /// The timestamp associated with the capability.
     fn time(&self) -> &T;
-    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool;
+    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>, port: usize) -> bool;
 }
 
 impl<T: Timestamp, C: CapabilityTrait<T>> CapabilityTrait<T> for &C {
     fn time(&self) -> &T { (**self).time() }
-    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
-        (**self).valid_for_output(query_buffer)
+    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>, port: usize) -> bool {
+        (**self).valid_for_output(query_buffer, port)
     }
 }
 impl<T: Timestamp, C: CapabilityTrait<T>> CapabilityTrait<T> for &mut C {
     fn time(&self) -> &T { (**self).time() }
-    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
-        (**self).valid_for_output(query_buffer)
+    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>, port: usize) -> bool {
+        (**self).valid_for_output(query_buffer, port)
     }
 }
 
@@ -66,7 +66,7 @@ pub struct Capability<T: Timestamp> {
 
 impl<T: Timestamp> CapabilityTrait<T> for Capability<T> {
     fn time(&self) -> &T { &self.time }
-    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
+    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>, _port: usize) -> bool {
         Rc::ptr_eq(&self.internal, query_buffer)
     }
 }
@@ -227,9 +227,9 @@ impl Error for DowngradeError {}
 /// A shared list of shared output capability buffers.
 type CapabilityUpdates<T> = Rc<RefCell<Vec<Rc<RefCell<ChangeBatch<T>>>>>>;
 
-/// An capability of an input port. 
+/// An capability of an input port.
 ///
-/// Holding onto this capability will implicitly holds onto a capability for all the outputs 
+/// Holding onto this capability will implicitly holds onto a capability for all the outputs
 /// ports this input is connected to, after the connection summaries have been applied.
 ///
 /// This input capability supplies a `retain_for_output(self)` method which consumes the input
@@ -245,14 +245,12 @@ pub struct InputCapability<T: Timestamp> {
 
 impl<T: Timestamp> CapabilityTrait<T> for InputCapability<T> {
     fn time(&self) -> &T { self.time() }
-    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
+    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>, port: usize) -> bool {
         let summaries_borrow = self.summaries.borrow();
         let internal_borrow = self.internal.borrow();
         // To be valid, the output buffer must match and the timestamp summary needs to be the default.
-        let result = summaries_borrow.iter_ports().any(|(port, path)| {
-            Rc::ptr_eq(&internal_borrow[port], query_buffer) && path.len() == 1 && path[0] == Default::default()
-        });
-        result
+        Rc::ptr_eq(&internal_borrow[port], query_buffer) &&
+        summaries_borrow.get(port).map_or(false, |path| path.elements() == [Default::default()])
     }
 }
 
@@ -353,8 +351,8 @@ pub struct ActivateCapability<T: Timestamp> {
 
 impl<T: Timestamp> CapabilityTrait<T> for ActivateCapability<T> {
     fn time(&self) -> &T { self.capability.time() }
-    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
-        self.capability.valid_for_output(query_buffer)
+    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>, port: usize) -> bool {
+        self.capability.valid_for_output(query_buffer, port)
     }
 }
 
diff --git a/timely/src/dataflow/operators/generic/builder_rc.rs b/timely/src/dataflow/operators/generic/builder_rc.rs
index 66957ddf5..c3d02a780 100644
--- a/timely/src/dataflow/operators/generic/builder_rc.rs
+++ b/timely/src/dataflow/operators/generic/builder_rc.rs
@@ -127,7 +127,7 @@ impl<G: Scope> OperatorBuilder<G> {
             self.summaries[input].borrow_mut().add_port(new_output, entry);
         }
 
-        (OutputWrapper::new(buffer, internal), stream)
+        (OutputWrapper::new(buffer, internal, new_output), stream)
     }
 
     /// Creates an operator implementation from supplied logic constructor.
diff --git a/timely/src/dataflow/operators/generic/handles.rs b/timely/src/dataflow/operators/generic/handles.rs
index 154ebeddb..16d8a149c 100644
--- a/timely/src/dataflow/operators/generic/handles.rs
+++ b/timely/src/dataflow/operators/generic/handles.rs
@@ -169,14 +169,16 @@ pub fn new_input_handle<T: Timestamp, C: Container, P: Pull<Message<T, C>>>(
 pub struct OutputWrapper<T: Timestamp, CB: ContainerBuilder, P: Push<Message<T, CB::Container>>> {
     push_buffer: Buffer<T, CB, PushCounter<T, CB::Container, P>>,
     internal_buffer: Rc<RefCell<ChangeBatch<T>>>,
+    port: usize,
 }
 
 impl<T: Timestamp, CB: ContainerBuilder, P: Push<Message<T, CB::Container>>> OutputWrapper<T, CB, P> {
     /// Creates a new output wrapper from a push buffer.
-    pub fn new(push_buffer: Buffer<T, CB, PushCounter<T, CB::Container, P>>, internal_buffer: Rc<RefCell<ChangeBatch<T>>>) -> Self {
+    pub fn new(push_buffer: Buffer<T, CB, PushCounter<T, CB::Container, P>>, internal_buffer: Rc<RefCell<ChangeBatch<T>>>, port: usize) -> Self {
         OutputWrapper {
             push_buffer,
             internal_buffer,
+            port,
         }
     }
     /// Borrows the push buffer into a handle, which can be used to send records.
@@ -187,6 +189,7 @@ impl<T: Timestamp, CB: ContainerBuilder, P: Push<Message<T, CB::Container>>> Out
         OutputHandleCore {
             push_buffer: &mut self.push_buffer,
             internal_buffer: &self.internal_buffer,
+            port: self.port,
         }
     }
 }
@@ -195,6 +198,7 @@ impl<T: Timestamp, CB: ContainerBuilder, P: Push<Message<T, CB::Container>>> Out
 pub struct OutputHandleCore<'a, T: Timestamp, CB: ContainerBuilder+'a, P: Push<Message<T, CB::Container>>+'a> {
     push_buffer: &'a mut Buffer<T, CB, PushCounter<T, CB::Container, P>>,
     internal_buffer: &'a Rc<RefCell<ChangeBatch<T>>>,
+    port: usize,
 }
 
 /// Handle specialized to `Vec`-based container.
@@ -225,7 +229,7 @@ impl<'a, T: Timestamp, CB: ContainerBuilder, P: Push<Message<T, CB::Container>>>
     /// });
     /// ```
     pub fn session_with_builder<'b, CT: CapabilityTrait<T>>(&'b mut self, cap: &'b CT) -> Session<'b, T, CB, PushCounter<T, CB::Container, P>> where 'a: 'b {
-        debug_assert!(cap.valid_for_output(self.internal_buffer), "Attempted to open output session with invalid capability");
+        debug_assert!(cap.valid_for_output(self.internal_buffer, self.port), "Attempted to open output session with invalid capability");
         self.push_buffer.session_with_builder(cap.time())
     }