diff --git a/pageserver/compaction/src/helpers.rs b/pageserver/compaction/src/helpers.rs
index 1b80373ba7..eb0e5ee82a 100644
--- a/pageserver/compaction/src/helpers.rs
+++ b/pageserver/compaction/src/helpers.rs
@@ -9,6 +9,7 @@ use pageserver_api::shard::ShardIdentity;
 use pin_project_lite::pin_project;
 use std::collections::BinaryHeap;
 use std::collections::VecDeque;
+use std::fmt::Display;
 use std::future::Future;
 use std::ops::{DerefMut, Range};
 use std::pin::Pin;
@@ -214,7 +215,7 @@ pub struct KeySize<K> {
 
 pub fn accum_key_values<'a, I, K, D, E>(input: I) -> impl Stream<Item = Result<KeySize<K>, E>>
 where
-    K: Eq,
+    K: Eq + PartialOrd + Display + Copy,
     I: Stream<Item = Result<D, E>>,
     D: CompactionDeltaEntry<'a, K>,
 {
@@ -229,12 +230,15 @@ where
                 num_values: 1,
                 size: first.size(),
             };
+            let mut last_key = accum.key;
             while let Some(this) = input.next().await {
                 let this = this?;
                 if this.key() == accum.key {
                     accum.size += this.size();
                     accum.num_values += 1;
                 } else {
+                    assert!(last_key <= accum.key, "last_key={last_key} <= accum.key={}", accum.key);
+                    last_key = accum.key;
                     yield accum;
                     accum = KeySize {
                         key: this.key(),
@@ -243,6 +247,7 @@ where
                     };
                 }
             }
+            assert!(last_key <= accum.key, "last_key={last_key} <= accum.key={}", accum.key);
             yield accum;
         }
     }