diff --git a/morphium-core/src/main/java/de/caluga/morphium/IndexDescription.java b/morphium-core/src/main/java/de/caluga/morphium/IndexDescription.java
index 672739c3f..146cbe7c8 100644
--- a/morphium-core/src/main/java/de/caluga/morphium/IndexDescription.java
+++ b/morphium-core/src/main/java/de/caluga/morphium/IndexDescription.java
@@ -45,10 +45,23 @@ public static IndexDescription fromMap(Map<String, Object> incoming) {
             @SuppressWarnings("unchecked")
             Map<String, Object> keymap = (Map<String, Object>) incoming.get("key");
             for (var k : keymap.keySet()) {
+                // MongoDB's own naming convention is "<field>_<direction>" per key, joined by
+                // "_" between entries -- there is no separator after the LAST entry. Appending
+                // "_" unconditionally after every entry (as this used to do) produces a
+                // trailing underscore ("campaignNumber_1_" instead of "campaignNumber_1"), which
+                // silently breaks index creation on any database where an index on the same
+                // field already exists under the correct name: MongoDB rejects the mismatched
+                // name with "Error 85 - Index already exists with a different name", Morphium
+                // only logs that as a warning, and the index (with any unique constraint) is
+                // never created. This bug predates 6.3.0 -- it is present unchanged as far back
+                // as the v6.2.5 tag -- so it is a plain bugfix, not a behaviour change requiring
+                // a migration path.
+                if (sb.length() > 0) {
+                    sb.append("_");
+                }
                 sb.append(k);
                 sb.append("_");
                 sb.append(keymap.get(k).toString());
-                sb.append("_");
             }
             incoming.put("name", sb.toString());
         }
diff --git a/morphium-core/src/test/java/de/caluga/test/mongo/suite/base/IndexDescriptionTest.java b/morphium-core/src/test/java/de/caluga/test/mongo/suite/base/IndexDescriptionTest.java
index 5d2a449a5..b6e9ffb1f 100644
--- a/morphium-core/src/test/java/de/caluga/test/mongo/suite/base/IndexDescriptionTest.java
+++ b/morphium-core/src/test/java/de/caluga/test/mongo/suite/base/IndexDescriptionTest.java
@@ -46,4 +46,30 @@ public void asMapFromMapTest() throws Exception {
         assertEquals(idx.getHidden(), idx2.getHidden());
         assertEquals(idx.getSparse(), idx2.getSparse());
     }
+
+    // Regression test: fromMap() used to append a trailing "_" separator after every key
+    // instead of only BETWEEN keys, producing names like "campaignNumber_1_" instead of the
+    // MongoDB-standard "campaignNumber_1". That mismatch breaks index creation on any database
+    // where the correctly-named index already exists (MongoDB rejects it with "Error 85 - Index
+    // already exists with a different name", which Morphium only logs as a warning). Neither
+    // pre-existing test above catches this: both set an explicit name, which skips the
+    // auto-naming branch entirely.
+    @Test
+    public void fromMap_singleField_generatesNameWithoutTrailingUnderscore() throws Exception {
+        var idx = IndexDescription.fromMaps(Doc.of("campaignNumber", 1), null);
+        assertEquals("campaignNumber_1", idx.getName());
+    }
+
+    @Test
+    public void fromMap_multiField_generatesNameJoinedByUnderscoreWithoutTrailingUnderscore() throws Exception {
+        var idx = IndexDescription.fromMaps(Doc.of("campaignNumber", 1, "fileName", 1), null);
+        assertEquals("campaignNumber_1_fileName_1", idx.getName());
+    }
+
+    @Test
+    public void fromMap_explicitName_isNotOverwritten() throws Exception {
+        var idx = IndexDescription.fromMaps(Doc.of("campaignNumber", 1),
+                Doc.of("name", "myCustomIndexName"));
+        assertEquals("myCustomIndexName", idx.getName());
+    }
 }