diff --git a/quinn-proto/src/connection/mod.rs b/quinn-proto/src/connection/mod.rs
index 9ba5446cc..69b3eaae5 100644
--- a/quinn-proto/src/connection/mod.rs
+++ b/quinn-proto/src/connection/mod.rs
@@ -1488,6 +1488,8 @@ impl Connection {
 
         // Send MTU probe if necessary
         if transmit.is_empty() && self.state.is_established() {
+            // MTU probing happens only in Data space.
+            let space_id = SpaceId::Data;
             path_id = *self.paths.first_key_value().expect("one path must exist").0;
             let probe_data = loop {
                 // We MTU probe all paths for which all of the following is true:
@@ -1501,9 +1503,7 @@ impl Connection {
                     && !self.abandoned_paths.contains(&path_id);
                 let probe_size = eligible
                     .then(|| {
-                        let next_pn = self.spaces[SpaceId::Data]
-                            .for_path(path_id)
-                            .peek_tx_number();
+                        let next_pn = self.spaces[space_id].for_path(path_id).peek_tx_number();
                         self.path_data_mut(path_id).mtud.poll_transmit(now, next_pn)
                     })
                     .flatten();
@@ -5089,6 +5089,11 @@ impl Connection {
 
         // IMMEDIATE_ACK
         if mem::replace(&mut space.for_path(path_id).immediate_ack_pending, false) {
+            debug_assert_eq!(
+                space_id,
+                SpaceId::Data,
+                "immediate acks must be sent in the data space"
+            );
             trace!("IMMEDIATE_ACK");
             buf.write(frame::FrameType::IMMEDIATE_ACK);
             sent.non_retransmits = true;
@@ -5882,6 +5887,11 @@ impl Connection {
     /// According to the spec, this will result in an error if the remote endpoint does not support
     /// the Acknowledgement Frequency extension
     pub(crate) fn immediate_ack(&mut self, path_id: PathId) {
+        debug_assert_eq!(
+            self.highest_space,
+            SpaceId::Data,
+            "immediate ack must be written in the data space"
+        );
         self.spaces[self.highest_space]
             .for_path(path_id)
             .immediate_ack_pending = true;