diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/ReftableTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/ReftableTest.java
index db9a4d0ac..fc363ecf2 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/ReftableTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/ReftableTest.java
@@ -46,13 +46,16 @@ package org.eclipse.jgit.internal.storage.reftable;
 import static org.eclipse.jgit.lib.Constants.HEAD;
 import static org.eclipse.jgit.lib.Constants.OBJECT_ID_LENGTH;
 import static org.eclipse.jgit.lib.Constants.R_HEADS;
+import static org.eclipse.jgit.lib.MoreAsserts.assertThrows;
 import static org.eclipse.jgit.lib.Ref.Storage.NEW;
 import static org.eclipse.jgit.lib.Ref.Storage.PACKED;
+import static org.hamcrest.CoreMatchers.containsString;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
@@ -414,6 +417,54 @@ public class ReftableTest {
 		assertSeek(refs, read(table));
 	}
 
+	@Test
+	public void invalidRefWriteOrder() throws IOException {
+		Ref master = ref(MASTER, 1);
+		Ref next = ref(NEXT, 2);
+		ReftableWriter writer = new ReftableWriter()
+			.setMinUpdateIndex(1)
+			.setMaxUpdateIndex(1)
+			.begin(new ByteArrayOutputStream());
+
+		writer.writeRef(next);
+		IllegalArgumentException e  = assertThrows(
+			IllegalArgumentException.class,
+			() -> writer.writeRef(master));
+		assertThat(e.getMessage(), containsString("records must be increasing"));
+	}
+
+	@Test
+	public void invalidReflogWriteOrderUpdateIndex() throws IOException {
+		ReftableWriter writer = new ReftableWriter()
+			.setMinUpdateIndex(1)
+			.setMaxUpdateIndex(2)
+			.begin(new ByteArrayOutputStream());
+		PersonIdent who = new PersonIdent("Log", "Ger", 1500079709, -8 * 60);
+		String msg = "test";
+
+		writer.writeLog(MASTER, 1, who, ObjectId.zeroId(), id(1), msg);
+		IllegalArgumentException e  = assertThrows(IllegalArgumentException.class,
+			() -> writer.writeLog(
+				MASTER, 2, who, ObjectId.zeroId(), id(2), msg));
+		assertThat(e.getMessage(), containsString("records must be increasing"));
+	}
+
+	@Test
+	public void invalidReflogWriteOrderName() throws IOException {
+		ReftableWriter writer = new ReftableWriter()
+			.setMinUpdateIndex(1)
+			.setMaxUpdateIndex(1)
+			.begin(new ByteArrayOutputStream());
+		PersonIdent who = new PersonIdent("Log", "Ger", 1500079709, -8 * 60);
+		String msg = "test";
+
+		writer.writeLog(NEXT, 1, who, ObjectId.zeroId(), id(1), msg);
+		IllegalArgumentException e = assertThrows(IllegalArgumentException.class,
+			() -> writer.writeLog(
+				MASTER, 1, who, ObjectId.zeroId(), id(2), msg));
+		assertThat(e.getMessage(), containsString("records must be increasing"));
+	}
+
 	@Test
 	public void withReflog() throws IOException {
 		Ref master = ref(MASTER, 1);
@@ -577,7 +628,7 @@ public class ReftableTest {
 
 		List<Ref> refs = new ArrayList<>();
 		for (int i = 1; i <= 5670; i++) {
-			Ref ref = ref(String.format("refs/heads/%03d", i), i);
+			Ref ref = ref(String.format("refs/heads/%04d", i), i);
 			refs.add(ref);
 			writer.writeRef(ref);
 		}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableWriter.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableWriter.java
index f8b9ffd17..726339617 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableWriter.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableWriter.java
@@ -44,6 +44,7 @@
 package org.eclipse.jgit.internal.storage.reftable;
 
 import static java.lang.Math.log;
+import static java.nio.charset.StandardCharsets.UTF_8;
 import static org.eclipse.jgit.internal.storage.reftable.BlockWriter.padBetweenBlocks;
 import static org.eclipse.jgit.internal.storage.reftable.ReftableConstants.FILE_FOOTER_LEN;
 import static org.eclipse.jgit.internal.storage.reftable.ReftableConstants.FILE_HEADER_LEN;
@@ -108,6 +109,8 @@ public class ReftableWriter {
 	private ReftableOutputStream out;
 	private ObjectIdSubclassMap<RefList> obj2ref;
 
+	private BlockWriter.Entry lastRef;
+	private BlockWriter.Entry lastLog;
 	private BlockWriter cur;
 	private Section refs;
 	private Section objs;
@@ -120,6 +123,8 @@ public class ReftableWriter {
 	 */
 	public ReftableWriter() {
 		this(new ReftableConfig());
+		lastRef = null;
+		lastLog = null;
 	}
 
 	/**
@@ -269,10 +274,22 @@ public class ReftableWriter {
 			throw new IllegalArgumentException();
 		}
 		long d = updateIndex - minUpdateIndex;
-		long blockPos = refs.write(new RefEntry(ref, d));
+		RefEntry entry = new RefEntry(ref, d);
+		if (lastRef != null && Entry.compare(lastRef, entry) >= 0) {
+			throwIllegalEntry(lastRef, entry);
+		}
+		lastRef = entry;
+
+		long blockPos = refs.write(entry);
 		indexRef(ref, blockPos);
 	}
 
+	private void throwIllegalEntry(Entry last, Entry now) {
+		throw new IllegalArgumentException(
+			String.format("records must be increasing: last %s, this %s",
+				new String(last.key, UTF_8), new String(now.key, UTF_8)));
+	}
+
 	private void indexRef(Ref ref, long blockPos) {
 		if (indexObjects && !ref.isSymbolic()) {
 			indexId(ref.getObjectId(), blockPos);
@@ -322,7 +339,12 @@ public class ReftableWriter {
 					throws IOException {
 		String msg = message != null ? message : ""; //$NON-NLS-1$
 		beginLog();
-		logs.write(new LogEntry(ref, updateIndex, who, oldId, newId, msg));
+		LogEntry entry = new LogEntry(ref, updateIndex, who, oldId, newId, msg);
+		if (lastLog != null && Entry.compare(lastLog, entry) >= 0) {
+			throwIllegalEntry(lastLog, entry);
+		}
+		lastLog = entry;
+		logs.write(entry);
 	}
 
 	/**