diff --git a/README.md b/README.md
index 6ee26e6..51b265c 100644
--- a/README.md
+++ b/README.md
@@ -28,7 +28,7 @@ import PackageDescription
 
 let package = Package(
     name: "MyApp",
-    dependencies: [.Package(url: "https://github.com/OpenKitten/BSON.git", majorVersion: 3, minor: 6)]
+    dependencies: [.Package(url: "https://github.com/OpenKitten/BSON.git", majorVersion: 3, minor: 7)]
 )
 ```
 
diff --git a/Sources/Document+Subscripts.swift b/Sources/Document+Subscripts.swift
index d38fe1b..7ed50ff 100644
--- a/Sources/Document+Subscripts.swift
+++ b/Sources/Document+Subscripts.swift
@@ -54,6 +54,7 @@ extension Document {
                 storage.insert(contentsOf: newValue.bytes, at: meta.dataPosition)
                 storage[meta.elementTypePosition] = newValue.typeIdentifier
                 updateDocumentHeader()
+                self.elementPositions = buildElementPositionsCache()
                 
                 return
             }
diff --git a/Sources/ObjectId.swift b/Sources/ObjectId.swift
index f547a2e..63d1fd6 100644
--- a/Sources/ObjectId.swift
+++ b/Sources/ObjectId.swift
@@ -7,6 +7,7 @@
 //
 
 import Foundation
+import Dispatch
 
 #if os(Linux)
     import Glibc
@@ -38,6 +39,8 @@ public struct ObjectId {
     
     internal var _storage: [UInt8]
     
+    private static let counterQueue = DispatchQueue(label: "org.mongokitten.bson.oidcounter")
+    
     /// Generate a new random ObjectId.
     public init() {
         let currentTime = Date()
@@ -51,8 +54,10 @@ public struct ObjectId {
         data += ObjectId.random.bytes
         
         // And add a counter as 2 bytes and increment it
-        data += ObjectId.counter.bytes
-        ObjectId.counter = ObjectId.counter &+ 1
+        ObjectId.counterQueue.sync {
+            data += ObjectId.counter.bytes
+            ObjectId.counter = ObjectId.counter &+ 1
+        }
         
         self._storage = data
     }