diff --git a/JsonFlatFileDataStore.Test/CollectionTests.cs b/JsonFlatFileDataStore.Test/CollectionTests.cs
index 1d67d82..89ce29f 100644
--- a/JsonFlatFileDataStore.Test/CollectionTests.cs
+++ b/JsonFlatFileDataStore.Test/CollectionTests.cs
@@ -56,10 +56,11 @@ public void GetNextIdValue_IntegerId()
             var nextId = collection.GetNextIdValue();
             Assert.Equal(4, nextId);
 
+            collection.InsertOne(new { id = nextId + 1 });
             collection.InsertOne(new { id = nextId });
 
             nextId = collection.GetNextIdValue();
-            Assert.Equal(5, nextId);
+            Assert.Equal(6, nextId);
         }
 
         [Fact]
@@ -124,7 +125,7 @@ public void GetNextIdValue_StringType_AnonymousType()
             collection.InsertOne(new { myId = "hello2" });
 
             nextId = collection.GetNextIdValue();
-            Assert.Equal("hello3", nextId);
+            Assert.Equal("hello5", nextId);
 
             var same = collection.AsQueryable().Where(e => e.myId == "hello2");
             Assert.Equal(2, same.Count());
@@ -140,17 +141,19 @@ public void GetNextIdValue_StringType_JToken()
             var collection = store.GetCollection("collectionWithStringId");
 
             // Insert seed value with upsert
-            collection.ReplaceOne(e => e, JToken.Parse("{ 'myId': 'test1' }"), true);
+            collection.ReplaceOne(e => true, JToken.Parse("{ 'myId': 'test1' }"), true);
+            collection.InsertOne(JToken.Parse("{ 'myId': 'test3' }"));
+            collection.InsertOne(JToken.Parse("{ 'myId': 'test2' }"));
 
             var nextId = collection.GetNextIdValue();
-            Assert.Equal("test2", nextId);
+            Assert.Equal("test4", nextId);
 
-            var nextUpdate = JToken.Parse("{ 'myId': 'somethingWrong2' }");
+            var nextUpdate = JToken.Parse("{ 'myId': 'somethingWrong4' }");
             collection.InsertOne(nextUpdate);
             Assert.Equal(nextId, nextUpdate["myId"]);
 
             nextId = collection.GetNextIdValue();
-            Assert.Equal("test3", nextId);
+            Assert.Equal("test5", nextId);
 
             nextUpdate = JToken.Parse("{ 'xxx': 111 }");
             collection.InsertOne(nextUpdate);
@@ -158,7 +161,7 @@ public void GetNextIdValue_StringType_JToken()
             Assert.Equal(111, nextUpdate["xxx"]);
 
             nextId = collection.GetNextIdValue();
-            Assert.Equal("test4", nextId);
+            Assert.Equal("test6", nextId);
         }
 
         [Fact]
diff --git a/JsonFlatFileDataStore/DocumentCollection.cs b/JsonFlatFileDataStore/DocumentCollection.cs
index 57ae392..7950ccd 100644
--- a/JsonFlatFileDataStore/DocumentCollection.cs
+++ b/JsonFlatFileDataStore/DocumentCollection.cs
@@ -372,20 +372,29 @@ private dynamic GetNextIdValue(List<T> data)
             if (!data.Any())
                 return 0;
 
-            var lastItem = data.Last();
-            var expando = JsonConvert.DeserializeObject<ExpandoObject>(JsonConvert.SerializeObject(lastItem), new ExpandoObjectConverter());
-            // Problem here is if we have typed data with upper camel case properties but lower camel case in JSON, so need to use OrdinalIgnoreCase string comparer
-            var expandoAsIgnoreCase = new Dictionary<string, dynamic>(expando, StringComparer.OrdinalIgnoreCase);
+            var idValues = data.Select(item =>
+            {
+                var expando = JsonConvert.DeserializeObject<ExpandoObject>(JsonConvert.SerializeObject(item), new ExpandoObjectConverter());
+                // Problem here is if we have typed data with upper camel case properties but lower camel case in JSON, so need to use OrdinalIgnoreCase string comparer
+                var expandoIgnoreCase = new Dictionary<string, dynamic>(expando, StringComparer.OrdinalIgnoreCase);
+
+                if (!expandoIgnoreCase.ContainsKey(_idField))
+                {
+                    return null;
+                }
+
+                return expandoIgnoreCase[_idField];
+            });
 
-            if (!expandoAsIgnoreCase.ContainsKey(_idField))
-                return null;
+            var maxIdValue = idValues.Max();
 
-            dynamic keyValue = expandoAsIgnoreCase[_idField];
+            if (maxIdValue == null)
+                return 0;
 
-            if (keyValue is Int64)
-                return (int)keyValue + 1;
+            if (maxIdValue is Int64)
+                return (int)maxIdValue + 1;
 
-            return ParseNextIntegertToKeyValue(keyValue.ToString());
+            return ParseNextIntegertToKeyValue(maxIdValue.ToString());
         }
 
         private string ParseNextIntegertToKeyValue(string input)