added ImageBackupNotAvailableException

src/Intervention/Image/Exception/ImageBackupNotAvailableException.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace Intervention\Image\Exception;
+
+class ImageBackupNotAvailableException extends \RuntimeException
+{
+    # nothing to override
+}

src/Intervention/Image/Image.php

@@ -1469,6 +1469,8 @@ class Image
         if ($this->isImageResource($this->original)) {
             is_resource($this->resource) ? imagedestroy($this->resource) : null;
             $this->initFromResource($this->original);
+        } else {
+            throw new Exception\ImageBackupNotAvailableException('backup() must be called first to use reset().');
         }
 
         return $this;

tests/ImageTest.php

@@ -1419,6 +1419,15 @@ class ImageTest extends PHPUnit_Framework_Testcase
         $this->assertEquals('#0000ff', $img->pickColor(0, 0, 'hex'));
     }
 
+    /**
+     * @expectedException Intervention\Image\Exception\ImageBackupNotAvailableException
+     */
+    public function testResetImageWithoutBackup()
+    {
+        $img = $this->getTestImage();
+        $img->reset();
+    }
+
     public function testBackup()
     {
         $img = new Image(null, 800, 600, '#0000ff');