Index: oak-store-document/src/main/java/org/apache/jackrabbit/oak/plugins/document/rdb/RDBDocumentSerializer.java
===================================================================
--- oak-store-document/src/main/java/org/apache/jackrabbit/oak/plugins/document/rdb/RDBDocumentSerializer.java	(revision 1844639)
+++ oak-store-document/src/main/java/org/apache/jackrabbit/oak/plugins/document/rdb/RDBDocumentSerializer.java	(working copy)
@@ -232,6 +232,9 @@
             }
             json.read(JsopReader.END);
 
+            // OAK-7855: check and fix _sdType
+            checkSdType(doc);
+
             return doc;
         } catch (Exception ex) {
             String message = String.format("Error processing persisted data for document '%s'", row.getId());
@@ -321,6 +324,17 @@
         }
     }
 
+    private static void checkSdType(Document doc) {
+        Object sdType = doc.get(NodeDocument.SD_TYPE);
+        if (sdType instanceof Long) {
+            long value = (long) sdType;
+            if (value == 0) {
+                doc.remove(NodeDocument.SD_TYPE);
+                LOG.debug("Incorrect _sdType 0 in {}", doc.getId());
+            }
+        }
+    }
+
     // low level operations
 
     private static byte[] GZIPSIG = { 31, -117 };