diff --git a/src/DataFormatter/JSONDataFormatter.php b/src/DataFormatter/JSONDataFormatter.php
index b877832..c5a784a 100644
--- a/src/DataFormatter/JSONDataFormatter.php
+++ b/src/DataFormatter/JSONDataFormatter.php
@@ -9,6 +9,7 @@ use SilverStripe\RestfulServer\DataFormatter;
 use SilverStripe\ORM\DataObjectInterface;
 use SilverStripe\Control\Director;
 use SilverStripe\ORM\SS_List;
+use SilverStripe\ORM\FieldType;
 
 /**
  * Formats a DataObject's member fields into a JSON string
@@ -89,7 +90,7 @@ class JSONDataFormatter extends DataFormatter
                 continue;
             }
 
-            $fieldValue = $obj->obj($fieldName)->RAW();
+            $fieldValue = self::cast($obj->obj($fieldName));
             $mappedFieldName = $this->getFieldAlias($className, $fieldName);
             $serobj->$mappedFieldName = $fieldValue;
         }
@@ -117,7 +118,7 @@ class JSONDataFormatter extends DataFormatter
                 $serobj->$relName = ArrayData::array_to_object(array(
                     "className" => $relClass,
                     "href" => "$href.json",
-                    "id" => $obj->$fieldName
+                    "id" => self::cast($obj->obj($fieldName))
                 ));
             }
 
@@ -190,4 +191,15 @@ class JSONDataFormatter extends DataFormatter
     {
         return Convert::json2array($strData);
     }
+
+    public static function cast(FieldType\DBField $dbfield)
+    {
+        if (is_null($dbfield->RAW())) return null;
+        switch (true) {
+            case $dbfield instanceof FieldType\DBInt: return (int)$dbfield->RAW();
+            case $dbfield instanceof FieldType\DBFloat: return (float)$dbfield->RAW();
+            case $dbfield instanceof FieldType\DBBoolean: return (bool)$dbfield->RAW();
+        }
+        return $dbfield->RAW();
+    }
 }
diff --git a/tests/unit/JSONDataFormatterTest.php b/tests/unit/JSONDataFormatterTest.php
new file mode 100644
index 0000000..a97f1e8
--- /dev/null
+++ b/tests/unit/JSONDataFormatterTest.php
@@ -0,0 +1,46 @@
+<?php
+
+namespace SilverStripe\RestfulServer\Tests;
+
+use SilverStripe\RestfulServer\RestfulServer;
+use SilverStripe\RestfulServer\Tests\Stubs\JSONDataFormatterTypeTestObject;
+use SilverStripe\Dev\SapphireTest;
+use SilverStripe\RestfulServer\DataFormatter\JSONDataFormatter;
+
+/**
+ *
+ * @todo Test Relation getters
+ * @todo Test filter and limit through GET params
+ * @todo Test DELETE verb
+ *
+ */
+class JSONDataFormatterTest extends SapphireTest
+{
+    protected static $fixture_file = 'JSONDataFormatterTest.yml';
+
+    protected static $extra_dataobjects = [
+        JSONDataFormatterTypeTestObject::class,
+    ];
+
+    public function testJSONTypes()
+    {
+        $formatter = new JSONDataFormatter();
+        $parent = $this->objFromFixture(JSONDataFormatterTypeTestObject::class, 'parent');
+        $json = $formatter->convertDataObject($parent);
+        $this->assertRegexp('/"ID":\d+/', $json, 'PK casted to integer');
+        $this->assertRegexp('/"Created":"\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}"/', $json, 'Datetime casted to ISO string');
+        $this->assertContains('"Name":"Parent"', $json, 'String casted to string');
+        $this->assertContains('"Active":true', $json, 'Boolean casted to boolean');
+        $this->assertContains('"Sort":17', $json, 'Integer casted to integer');
+        $this->assertContains('"Average":1.2345', $json, 'Float casted to float');
+        $this->assertContains('"ParentID":0', $json, 'Empty FK is 0');
+
+        $child3 = $this->objFromFixture(JSONDataFormatterTypeTestObject::class, 'child3');
+        $json = $formatter->convertDataObject($child3);
+        $this->assertContains('"Name":null', $json, 'Empty string is null');
+        $this->assertContains('"Active":false', $json, 'Empty boolean is false');
+        $this->assertContains('"Sort":0', $json, 'Empty integer is 0');
+        $this->assertContains('"Average":null', $json, 'Empty float is null');
+        $this->assertRegexp('/"ParentID":\d+/', $json, 'FK casted to integer');
+    }
+}
diff --git a/tests/unit/JSONDataFormatterTest.yml b/tests/unit/JSONDataFormatterTest.yml
new file mode 100644
index 0000000..4c6f2f8
--- /dev/null
+++ b/tests/unit/JSONDataFormatterTest.yml
@@ -0,0 +1,20 @@
+SilverStripe\RestfulServer\Tests\Stubs\JSONDataFormatterTypeTestObject:
+  parent:
+    Name: Parent
+    Active: true
+    Sort: 17
+    Average: 1.2345
+  child1:
+    Name: Child 1
+    Active: 1
+    Sort: 4
+    Average: 6.78
+    Parent: =>SilverStripe\RestfulServer\Tests\Stubs\JSONDataFormatterTypeTestObject.parent
+  child2:
+    Name: Child 2
+    Active: false
+    Sort: 9
+    Average: 1
+    Parent: =>SilverStripe\RestfulServer\Tests\Stubs\JSONDataFormatterTypeTestObject.parent
+  child3:
+    Parent: =>SilverStripe\RestfulServer\Tests\Stubs\JSONDataFormatterTypeTestObject.parent
diff --git a/tests/unit/Stubs/JSONDataFormatterTypeTestObject.php b/tests/unit/Stubs/JSONDataFormatterTypeTestObject.php
new file mode 100644
index 0000000..99ca52b
--- /dev/null
+++ b/tests/unit/Stubs/JSONDataFormatterTypeTestObject.php
@@ -0,0 +1,32 @@
+<?php
+
+namespace SilverStripe\RestfulServer\Tests\Stubs;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+
+class JSONDataFormatterTypeTestObject extends DataObject implements TestOnly
+{
+    /**
+     * @var string
+     */
+    private static $table_name = 'JSONDataFormatterTypeTestObject';
+
+    /**
+     * @var array
+     */
+    private static $db = [
+        'Name' => 'Varchar',
+        'Active' => 'Boolean',
+        'Sort' => 'Int',
+        'Average' => 'Float',
+    ];
+
+    private static $has_one = [
+        'Parent' => JSONDataFormatterTypeTestObject::class,
+    ];
+
+    private static $has_many = [
+        'Children' => JSONDataFormatterTypeTestObject::class,
+    ];
+}