diff --git a/server/src/main/java/org/elasticsearch/cluster/metadata/MetaDataIndexUpgradeService.java b/server/src/main/java/org/elasticsearch/cluster/metadata/MetaDataIndexUpgradeService.java
index 52ee0be49ef6..9c4696bfa70f 100644
--- a/server/src/main/java/org/elasticsearch/cluster/metadata/MetaDataIndexUpgradeService.java
+++ b/server/src/main/java/org/elasticsearch/cluster/metadata/MetaDataIndexUpgradeService.java
@@ -22,6 +22,7 @@
 import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.apache.lucene.analysis.Analyzer;
+import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarities.Similarity;
 import org.elasticsearch.Version;
 import org.elasticsearch.common.TriFunction;
@@ -145,7 +146,7 @@ public boolean containsKey(Object key) {
                 @Override
                 public TriFunction<Settings, Version, ScriptService, Similarity> get(Object key) {
                     assert key instanceof String : "key must be a string but was: " + key.getClass();
-                    return SimilarityService.BUILT_IN.get(SimilarityService.DEFAULT_SIMILARITY);
+                    return (settings, version, scriptService) -> new BM25Similarity();
                 }
 
                 // this entrySet impl isn't fully correct but necessary as SimilarityService will iterate
diff --git a/server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataIndexUpgradeServiceTests.java b/server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataIndexUpgradeServiceTests.java
index 32e7a3bcbd87..20deb39c7232 100644
--- a/server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataIndexUpgradeServiceTests.java
+++ b/server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataIndexUpgradeServiceTests.java
@@ -82,6 +82,18 @@ public void testUpgrade() {
         assertSame(src, service.upgradeIndexMetaData(src, Version.CURRENT.minimumIndexCompatibilityVersion())); // no double upgrade
     }
 
+    public void testUpgradeCustomSimilarity() {
+        MetaDataIndexUpgradeService service = getMetaDataIndexUpgradeService();
+        IndexMetaData src = newIndexMeta("foo",
+            Settings.builder()
+                .put("index.similarity.my_similarity.type", "DFR")
+                .put("index.similarity.my_similarity.after_effect", "l")
+                .build());
+        assertFalse(service.isUpgraded(src));
+        src = service.upgradeIndexMetaData(src, Version.CURRENT.minimumIndexCompatibilityVersion());
+        assertTrue(service.isUpgraded(src));
+    }
+
     public void testIsUpgraded() {
         MetaDataIndexUpgradeService service = getMetaDataIndexUpgradeService();
         IndexMetaData src = newIndexMeta("foo", Settings.builder().put("index.refresh_interval", "-200").build());