diff --git a/CHANGELOG.md b/CHANGELOG.md
index fc7a1b2..100a1db 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,24 @@
 # Changelog
+## dev-master
+- Spec compliance: error extensions are displayed under `extensions` key 
+
+#### v0.12.5
+- Execution performance optimization for lists
+
+#### v0.12.4
+- Allow stringeable objects to be serialized by StringType (#303)
+
+#### v0.12.3
+- StandardServer: add support for the multipart/form-data content type (#300)
+
+#### v0.12.2
+- SchemaPrinter: Use multi-line block for trailing quote (#294)
+
+#### v0.12.1
+- Fixed bug in validation rule OverlappingFieldsCanBeMerged (#292)
+- Added one more breaking change note in UPGRADE.md (#291)
+- Spec compliance: remove `data` entry from response on top-level error (#281)
+
 ## v0.12.0
 - RFC: Block String (multi-line strings via triple-quote """string""")
 - GraphQL Schema SDL: Descriptions as strings (including multi-line)
diff --git a/UPGRADE.md b/UPGRADE.md
index c9e2443..40b9d72 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -1,3 +1,40 @@
+## Upgrade v0.12.x > dev-master
+
+### Breaking: minimum supported version of PHP
+New minimum required version of PHP is **7.1+**
+
+### Breaking: errors formatting changed according to spec 
+Extensions assigned to errors are shown under `extensions` key
+```php
+$e = new Error(
+    'msg',
+    null,
+    null,
+    null,
+    null,
+    null,
+    ['foo' => 'bar']
+);
+```
+Formatting before the change:
+```
+'errors' => [
+    [
+        'message' => 'msg',
+        'foo' => 'bar'
+    ]
+]
+```
+After the change:
+```
+'errors' => [
+    [
+        'message' => 'msg',
+        'extensions' => ['foo' => 'bar'],
+    ]
+]
+```
+
 ## Upgrade v0.11.x > v0.12.x
 
 ### Breaking: Minimum supported version is PHP5.6
diff --git a/src/Error/Error.php b/src/Error/Error.php
index 1060161..02c26ec 100644
--- a/src/Error/Error.php
+++ b/src/Error/Error.php
@@ -326,10 +326,6 @@ class Error extends \Exception implements \JsonSerializable, ClientAware
             'message' => $this->getMessage(),
         ];
 
-        if ($this->getExtensions()) {
-            $arr = array_merge($this->getExtensions(), $arr);
-        }
-
         $locations = Utils::map(
             $this->getLocations(),
             function (SourceLocation $loc) {
@@ -343,6 +339,9 @@ class Error extends \Exception implements \JsonSerializable, ClientAware
         if (! empty($this->path)) {
             $arr['path'] = $this->path;
         }
+        if (! empty($this->extensions)) {
+            $arr['extensions'] = $this->extensions;
+        }
 
         return $arr;
     }
diff --git a/src/Error/FormattedError.php b/src/Error/FormattedError.php
index b73e641..25d364d 100644
--- a/src/Error/FormattedError.php
+++ b/src/Error/FormattedError.php
@@ -187,23 +187,21 @@ class FormattedError
         }
 
         if ($e instanceof Error) {
-            if ($e->getExtensions()) {
-                $formattedError = array_merge($e->getExtensions(), $formattedError);
-            }
-
             $locations = Utils::map(
                 $e->getLocations(),
                 function (SourceLocation $loc) {
                     return $loc->toSerializableArray();
                 }
             );
-
             if (! empty($locations)) {
                 $formattedError['locations'] = $locations;
             }
             if (! empty($e->path)) {
                 $formattedError['path'] = $e->path;
             }
+            if (! empty($e->getExtensions())) {
+                $formattedError['extensions'] = $e->getExtensions();
+            }
         }
 
         if ($debug) {
diff --git a/tests/Error/ErrorTest.php b/tests/Error/ErrorTest.php
index 8c98e7b..03abbad 100644
--- a/tests/Error/ErrorTest.php
+++ b/tests/Error/ErrorTest.php
@@ -146,6 +146,9 @@ class ErrorTest extends TestCase
         );
 
         $this->assertEquals(['foo' => 'bar'], $e->getExtensions());
-        $this->assertEquals(['message' => 'msg', 'foo' => 'bar'], $e->toSerializableArray());
+        $this->assertEquals([
+            'message' => 'msg',
+            'extensions' => ['foo' => 'bar'],
+        ], $e->toSerializableArray());
     }
 }
diff --git a/tests/Error/PrintErrorTest.php b/tests/Error/PrintErrorTest.php
index acb2428..8cff56c 100644
--- a/tests/Error/PrintErrorTest.php
+++ b/tests/Error/PrintErrorTest.php
@@ -5,12 +5,51 @@ use GraphQL\Error\Error;
 use GraphQL\Error\FormattedError;
 use GraphQL\Language\Parser;
 use GraphQL\Language\Source;
+use GraphQL\Language\SourceLocation;
 use PHPUnit\Framework\TestCase;
 
 class PrintErrorTest extends TestCase
 {
     // Describe printError
 
+    /**
+     * @it prints an line numbers with correct padding
+     */
+    public function testPrintsAnLineNumbersWithCorrectPadding()
+    {
+        $singleDigit = new Error(
+            'Single digit line number with no padding',
+            null,
+            new Source('*', 'Test', new SourceLocation(9, 1)),
+            [0]
+        );
+
+        $actual = FormattedError::printError($singleDigit);
+        $expected = 'Single digit line number with no padding
+
+Test (9:1)
+ 9: *
+    ^
+';
+        $this->assertEquals($expected, $actual);
+
+        $doubleDigit = new Error(
+            'Left padded first line number',
+            null,
+            new Source("*\n", 'Test', new SourceLocation(9, 1)),
+            [0]
+        );
+        $actual = FormattedError::printError($doubleDigit);
+        $expected = 'Left padded first line number
+
+Test (9:1)
+ 9: *
+    ^
+10: 
+';
+        $this->assertEquals($expected, $actual);
+    }
+
     /**
      * @it prints an error with nodes from different sources
      */