diff --git a/code/libraries/array/java/nu/marginalia/array/page/LongQueryBuffer.java b/code/libraries/array/java/nu/marginalia/array/page/LongQueryBuffer.java
index 35421f39b..6858bdb31 100644
--- a/code/libraries/array/java/nu/marginalia/array/page/LongQueryBuffer.java
+++ b/code/libraries/array/java/nu/marginalia/array/page/LongQueryBuffer.java
@@ -115,6 +115,13 @@ public class LongQueryBuffer {
         return ++read < end;
     }
 
+    public boolean isAscending() {
+        for (int i = read + 1; i < end; i++) {
+            if (data.get(i-1) > data.get(i))
+                return false;
+        }
+        return true;
+    }
     /** Retains the current value at the read pointer and advances the read and write pointers.
      *  Returns true if there are more values to read.
      *  <p></p> To enable "or" style criterias, the method swaps the current value with the value
diff --git a/code/libraries/skiplist/java/nu/marginalia/skiplist/SkipListReader.java b/code/libraries/skiplist/java/nu/marginalia/skiplist/SkipListReader.java
index 269722306..c1913da4a 100644
--- a/code/libraries/skiplist/java/nu/marginalia/skiplist/SkipListReader.java
+++ b/code/libraries/skiplist/java/nu/marginalia/skiplist/SkipListReader.java
@@ -10,6 +10,7 @@ import org.jetbrains.annotations.NotNull;
 import java.lang.foreign.MemorySegment;
 import java.lang.foreign.ValueLayout;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 
 public class SkipListReader {
@@ -64,6 +65,8 @@ public class SkipListReader {
      * a single page, and return true if additional computation is available.
      */
     public boolean tryRetainData(@NotNull LongQueryBuffer data) {
+        assert data.isAscending();
+
         try (var page = pool.get(currentBlock)) {
 
             int n = headerNumRecords(page, currentBlockOffset);
@@ -108,6 +111,8 @@ public class SkipListReader {
      * exist in the skip list index.
      */
     public void retainData(@NotNull LongQueryBuffer data) {
+        assert data.isAscending();
+
         while (data.hasMore()) {
             try (var page = pool.get(currentBlock)) {
 
@@ -198,6 +203,12 @@ public class SkipListReader {
         int pos = 0;
         long[] vals = new long[keys.length];
 
+        if (getClass().desiredAssertionStatus()) {
+            for (int i = 1; i < keys.length; i++) {
+                assert keys[i] >= keys[i-1] : "Not ascending: " + Arrays.toString(keys);
+            }
+        }
+
         while (pos < keys.length) {
             try (var page = pool.get(currentBlock)) {
                 MemorySegment ms = page.getMemorySegment();
@@ -290,6 +301,8 @@ public class SkipListReader {
      * a single page, and return true if additional computation is available.
      */
     public boolean tryRejectData(@NotNull LongQueryBuffer data) {
+        assert data.isAscending();
+
         try (var page = pool.get(currentBlock)) {
 
             int n = headerNumRecords(page, currentBlockOffset);
@@ -427,6 +440,7 @@ public class SkipListReader {
     public int getKeys(@NotNull LongQueryBuffer dest)
     {
         if (atEnd) return 0;
+        assert dest.isAscending();
 
         int totalCopied = 0;
         while (dest.fitsMore() && !atEnd) {