diff --git a/core/src/main/java/org/elasticsearch/index/engine/Engine.java b/core/src/main/java/org/elasticsearch/index/engine/Engine.java
index dea6390669f..0e610d68735 100644
--- a/core/src/main/java/org/elasticsearch/index/engine/Engine.java
+++ b/core/src/main/java/org/elasticsearch/index/engine/Engine.java
@@ -936,9 +936,6 @@ public abstract class Engine implements Closeable {
 
         public Operation(Term uid, long seqNo, long version, VersionType versionType, Origin origin, long startTime) {
             this.uid = uid;
-            // nocommit move these to InternalEngine where we can assert on the engine version
-            assert origin != Origin.PRIMARY || seqNo == SequenceNumbersService.UNASSIGNED_SEQ_NO : "seqNo should not be set when origin is PRIMARY";
-            assert origin == Origin.PRIMARY || seqNo >= 0 : "seqNo should be set when origin is not PRIMARY";
             this.seqNo = seqNo;
             this.version = version;
             this.versionType = versionType;
diff --git a/core/src/main/java/org/elasticsearch/index/engine/InternalEngine.java b/core/src/main/java/org/elasticsearch/index/engine/InternalEngine.java
index 632e1605c56..cf7f6f0fb99 100644
--- a/core/src/main/java/org/elasticsearch/index/engine/InternalEngine.java
+++ b/core/src/main/java/org/elasticsearch/index/engine/InternalEngine.java
@@ -528,7 +528,21 @@ public class InternalEngine extends Engine {
         return false;
     }
 
+    private boolean assertSequenceNumber(final Engine.Operation.Origin origin, final long seqNo) {
+        if (engineConfig.getIndexSettings().getIndexVersionCreated().before(Version.V_6_0_0_alpha1) && origin == Operation.Origin.LOCAL_TRANSLOG_RECOVERY) {
+            // legacy support
+            return seqNo == SequenceNumbersService.UNASSIGNED_SEQ_NO;
+        } else if (origin == Operation.Origin.PRIMARY) {
+            // sequence number should not be set when operation origin is primary
+            return seqNo == SequenceNumbersService.UNASSIGNED_SEQ_NO;
+        } else {
+            // sequence number should be set when operation origin is not primary
+            return seqNo >= 0;
+        }
+    }
+
     private IndexResult innerIndex(Index index) throws IOException {
+        assert assertSequenceNumber(index.origin(), index.seqNo());
         final Translog.Location location;
         final long updatedVersion;
         IndexResult indexResult = null;
@@ -692,6 +706,7 @@ public class InternalEngine extends Engine {
     }
 
     private DeleteResult innerDelete(Delete delete) throws IOException {
+        assert assertSequenceNumber(delete.origin(), delete.seqNo());
         final Translog.Location location;
         final long updatedVersion;
         final boolean found;
diff --git a/core/src/test/java/org/elasticsearch/bwcompat/OldIndexBackwardsCompatibilityIT.java b/core/src/test/java/org/elasticsearch/bwcompat/OldIndexBackwardsCompatibilityIT.java
index 6113f7e5aac..0ad8c2dfff1 100644
--- a/core/src/test/java/org/elasticsearch/bwcompat/OldIndexBackwardsCompatibilityIT.java
+++ b/core/src/test/java/org/elasticsearch/bwcompat/OldIndexBackwardsCompatibilityIT.java
@@ -91,9 +91,6 @@ import static org.hamcrest.Matchers.greaterThanOrEqualTo;
 // needs at least 2 nodes since it bumps replicas to 1
 @ESIntegTestCase.ClusterScope(scope = ESIntegTestCase.Scope.TEST, numDataNodes = 0)
 @LuceneTestCase.SuppressFileSystems("ExtrasFS")
-@LuceneTestCase.AwaitsFix(bugUrl = "needs a solution for translog operations that are recovered from the translog, don't have a seq no " +
-    "and trigger assertions in Engine.Operation")
-// nocommit. Fix ^^^ please.
 public class OldIndexBackwardsCompatibilityIT extends ESIntegTestCase {
     // TODO: test for proper exception on unsupported indexes (maybe via separate test?)
     // We have a 0.20.6.zip etc for this.
diff --git a/core/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java b/core/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
index ce41b251fea..727c15e1b34 100644
--- a/core/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
+++ b/core/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
@@ -166,7 +166,6 @@ import static org.hamcrest.Matchers.nullValue;
 
 public class InternalEngineTests extends ESTestCase {
 
-
     protected final ShardId shardId = new ShardId(new Index("index", "_na_"), 1);
     private static final IndexSettings INDEX_SETTINGS = IndexSettingsModule.newIndexSettings("index", Settings.EMPTY);
 
@@ -1207,7 +1206,7 @@ public class InternalEngineTests extends ESTestCase {
             assertThat(result.getFailure(), instanceOf(IllegalArgumentException.class));
             assertThat(result.getFailure().getMessage(), containsString("version type [FORCE] may not be used for non-translog operations"));
 
-            index = new Engine.Index(newUid("1"), doc, randomIntBetween(0, 16), 84, VersionType.FORCE,
+            index = new Engine.Index(newUid("1"), doc, SequenceNumbersService.UNASSIGNED_SEQ_NO, 84, VersionType.FORCE,
                     Engine.Operation.Origin.LOCAL_TRANSLOG_RECOVERY, 0, -1, false);
             result = engine.index(index);
             assertThat(result.getVersion(), equalTo(84L));
@@ -2086,8 +2085,6 @@ public class InternalEngineTests extends ESTestCase {
         return new Mapping(Version.CURRENT, root, new MetadataFieldMapper[0], emptyMap());
     }
 
-    // nocommit
-    @AwaitsFix(bugUrl = "trips assertions in Engine, assertions should be moved to InternalEngine")
     public void testUpgradeOldIndex() throws IOException {
         List<Path> indexes = new ArrayList<>();
         try (DirectoryStream<Path> stream = Files.newDirectoryStream(getBwcIndicesPath(), "index-*.zip")) {