diff -u b/core/lib/Drupal/Core/TypedData/Plugin/DataType/Map.php b/core/lib/Drupal/Core/TypedData/Plugin/DataType/Map.php
--- b/core/lib/Drupal/Core/TypedData/Plugin/DataType/Map.php
+++ b/core/lib/Drupal/Core/TypedData/Plugin/DataType/Map.php
@@ -114,8 +114,7 @@
         $value = $this->values[$property_name];
       }
       // If the property is unknown, this will throw an exception.
-      $this->properties[$property_name] = $this->getTypedDataManager()
-        ->getPropertyInstance($this, $property_name, $value);
+      $this->properties[$property_name] = $this->getTypedDataManager()->getPropertyInstance($this, $property_name, $value);
     }
     return $this->properties[$property_name];
   }
@@ -165,13 +164,16 @@
         }
       }
     }
-    //If module creator send an array to map dataType and don't setPropertyDefinition,
-    //Auto creating the PropertyDefinition so that it can be normalized
+    // If module creator send an array to map dataType and don't define setPropertyDefinition,
+    // Auto creating the PropertyDefinition so that it can   be normalized
     elseif (get_class($this->getDataDefinition()) == 'Drupal\Core\TypedData\MapDataDefinition') {
       $values = $this->values;
       if (is_array($values)) {
         foreach ($values as $key => $value) {
           $properties[$key] = \Drupal::typedDataManager()->create(
+            // Give array's value a DataType,So that drupal can find which normalizer to be used to normalize it
+            // Because we don't know the type of the value,so we use 'any',
+            // If you don't want to use 'any',Please define it by yourself use setPropertyDefinition
             DataDefinition::create('any'),
             $value,
             $key