diff --git a/convex-core/src/main/java/convex/core/data/AIndex.java b/convex-core/src/main/java/convex/core/data/AIndex.java
index c355e8997..d4fc00dc8 100644
--- a/convex-core/src/main/java/convex/core/data/AIndex.java
+++ b/convex-core/src/main/java/convex/core/data/AIndex.java
@@ -1,7 +1,7 @@
 package convex.core.data;
 
 import java.util.Collections;
-import java.util.HashSet;
+import java.util.LinkedHashSet;
 import java.util.Set;
 
 import convex.core.data.type.AType;
@@ -49,9 +49,13 @@ public boolean containsKey(ACell key) {
 	 */
 	public abstract V get(K key);
 
+	// entryAt(i) walks the radix tree in ascending key order, so a
+	// LinkedHashSet (which preserves insertion order) is needed here to keep
+	// that ordering visible through entrySet() — a plain HashSet iterates in
+	// hash-bucket order and silently discards it (#651).
 	@Override
 	public Set<Entry<K, V>> entrySet() {
-		HashSet<Entry<K,V>> hs=new HashSet<>(size());
+		LinkedHashSet<Entry<K,V>> hs=new LinkedHashSet<>(size());
 		long n=count();
 		for (long i=0; i<n; i++) {
 			MapEntry<K,V> me=entryAt(i);
diff --git a/convex-core/src/test/java/convex/core/data/IndexTest.java b/convex-core/src/test/java/convex/core/data/IndexTest.java
index 6e08a1242..0f62ffd20 100644
--- a/convex-core/src/test/java/convex/core/data/IndexTest.java
+++ b/convex-core/src/test/java/convex/core/data/IndexTest.java
@@ -235,6 +235,27 @@ public void testSymbolicKeys() throws InvalidDataException {
 		doIndexTests(rm);
 	}
 	
+	/**
+	 * entrySet() must preserve the Index's sorted key order (#651) — the class
+	 * is documented as "a sorted radix-tree map... provide sorted orderings for
+	 * indexes", and callers commonly iterate entrySet() (e.g. `for (var e :
+	 * index.entrySet())`) expecting that guarantee to hold, not just entryAt(i).
+	 */
+	@Test
+	public void testEntrySetOrder() throws InvalidDataException {
+		Index<ABlob, CVMLong> m = Index.none();
+		for (int i = 0; i < 50; i++) {
+			m = m.assoc(LongBlob.create(i), RT.cvm((long) i));
+		}
+		assertEquals(50L, m.count());
+
+		java.util.Iterator<java.util.Map.Entry<ABlob, CVMLong>> it = m.entrySet().iterator();
+		for (long i = 0; i < m.count(); i++) {
+			assertEquals(m.entryAt(i), it.next(), "entrySet() order diverged from entryAt() sorted order");
+		}
+		assertFalse(it.hasNext());
+	}
+
 	@Test public void testContains() {
 		Index<ABlob, CVMLong> bm=Samples.INT_INDEX_256;
 		long n=bm.count;