diff --git a/src/MockCollection.php b/src/MockCollection.php
index a8790ff..7802192 100644
--- a/src/MockCollection.php
+++ b/src/MockCollection.php
@@ -182,8 +182,18 @@ private function updateUpsert($filter, $update, $options, $anyUpdates)
     {
         if (array_key_exists('upsert', $options)) {
             if ($options['upsert'] && !$anyUpdates) {
-                if (array_key_exists('$set', $update)) {
-                    $documents = [array_merge($filter, $update['$set'])];
+                $set = $update['$set'] ?? [];
+                $setOnInsert = $update['$setOnInsert'] ?? [];
+                $commonFields = array_intersect_key($set, $setOnInsert);
+
+                //Emulate Mongo behaviour
+                //@link https://stackoverflow.com/questions/23992723/findandmodify-fails-with-error-cannot-update-field1-and-field1-at-the-same/23994425#23994425
+                if(!empty($commonFields)){
+                    throw new Exception('Operators $setOnInsert and $set cannot have the same field(s): '.
+                        implode(', ', array_keys($commonFields)));
+                }
+                if (!empty($set) || !empty($setOnInsert)) {
+                    $documents = [array_merge($filter, $set, $setOnInsert)];
                 } else {
                     $documents = [$update];
                 }
@@ -196,25 +206,27 @@ private function updateCore(&$doc, $update)
     {
         // The update operators are required, as exemplified here:
         // http://mongodb.github.io/mongo-php-library/tutorial/crud/
-        $supported = ['$set', '$unset', '$inc', '$push'];
+        $supported = ['$set', '$unset', '$inc', '$push', '$setOnInsert'];
         $unsupported = array_diff(array_keys($update), $supported);
         if (count($unsupported) > 0) {
             throw new Exception("Unsupported update operators found: " . implode(', ', $unsupported));
         }
 
-        foreach ($update['$set'] ?? [] as $k => $v) {
-            $dot = strpos($k, ".");
-            if ($dot !== false) {
-                $tmp = &$doc;
-                $keys = explode(".", $k);
-                if ($keys !== null) {
-                    foreach ($keys as $key) {
-                        $tmp = &$tmp[$key];
+        foreach(['$set', '$setOnInsert'] as $operator){
+            foreach ($update[$operator] ?? [] as $k => $v) {
+                $dot = strpos($k, ".");
+                if ($dot !== false) {
+                    $tmp = &$doc;
+                    $keys = explode(".", $k);
+                    if ($keys !== null) {
+                        foreach ($keys as $key) {
+                            $tmp = &$tmp[$key];
+                        }
+                        $tmp = $v;
                     }
-                    $tmp = $v;
+                } else {
+                    $doc[$k] = $v;
                 }
-            } else {
-                $doc[$k] = $v;
             }
         }
 
@@ -225,7 +237,11 @@ private function updateCore(&$doc, $update)
         }
 
         foreach ($update['$inc'] ?? [] as $k => $v) {
-            if (isset($doc[$k]) && is_integer($v) && is_integer($doc[$k])) {
+            //Mongo behaviour: it increments even a non-existing field
+            if(!isset($doc[$k])){
+                $doc[$k] = 0;
+            }
+            if (is_integer($v) && is_integer($doc[$k])) {
                 $doc[$k] += $v;
             }
         }
diff --git a/tests/MockCollectionTest.php b/tests/MockCollectionTest.php
index fcc1d73..d9f11cb 100644
--- a/tests/MockCollectionTest.php
+++ b/tests/MockCollectionTest.php
@@ -570,13 +570,13 @@ public function updateUpsertCore($x1, $x2, $x3)
 
         $this->col->updateOne(['bar' => 4], $x3, ['upsert' => true]);
         self::assertThat($this->col->count(['bar' => 1, 'foo' => 'Kekse']), self::equalTo(1));
-        if (array_key_exists('$set', $x3)) {
+        if (isset($x3['$set']) || isset($x3['$setOnInsert'])) {
             self::assertThat($this->col->count(['bar' => 1, 'foo' => 'bar']), self::equalTo(1));
         } else {
             self::assertThat($this->col->count(['bar' => 1, 'foo' => 'bar']), self::equalTo(2));
         }
         self::assertThat($this->col->count(['bar' => 2]), self::equalTo(1));
-        if (array_key_exists('$set', $x3)) {
+        if (isset($x3['$set']) || isset($x3['$setOnInsert'])) {
             self::assertThat($this->col->count(['bar' => 4]), self::equalTo(1));
         } else {
             self::assertThat($this->col->count(['bar' => 4]), self::equalTo(0));
@@ -596,6 +596,32 @@ public function testUpdateUpsertCanUpdateAndInsert()
         );
     }
 
+    /**
+     * @depends testInsertManyInsertsDocuments
+     */
+    public function testUpdateUpsertCanSetOnInsert()
+    {
+        $this->updateUpsertCore(
+            ['$setOnInsert' => ['foo' => 'Kekse']],
+            ['$setOnInsert' => ['foo' => 'Kekse']],
+            ['$setOnInsert' => ['foo' => 'bar']]
+        );
+    }
+
+    /**
+     * @depends testInsertManyInsertsDocuments
+     */
+    public function testUpdateUpsertException()
+    {
+        $this->expectException(Exception::class);
+        $this->updateUpsertCore(
+            ['$setOnInsert' => ['foo' => 'Kekse' ], '$set' => ['foo' => 'Kekse_']],
+            ['$setOnInsert' => ['foo' => 'Kekse' ], '$set' => ['foo' => 'Kekse_']],
+            ['$setOnInsert' => ['foo' => 'bar' ], '$set' => ['foo' => 'bar_']]
+        );
+    }
+
+
     /**
      * @depends testInsertManyInsertsDocuments
      */
@@ -608,6 +634,9 @@ public function testUpdateIncrement()
         $this->col->updateMany([], ['$inc' => ['bar' => 1]], ['upsert' => true]);
         self::assertThat($this->col->count(['bar' => 1]), self::equalTo(1));
 
+        $this->col->updateMany([], ['$inc' => ['none' => 1]], ['upsert' => true]);
+        self::assertThat($this->col->count(['none' => 1]), self::equalTo(1));
+
         $this->col->insertOne(
             ['foo' => 'foo', 'bar' => 1]
         );