diff --git a/parsers/ShortcodeParser.php b/parsers/ShortcodeParser.php
index 79ea1a52d..6123f9162 100644
--- a/parsers/ShortcodeParser.php
+++ b/parsers/ShortcodeParser.php
@@ -332,7 +332,21 @@ class ShortcodeParser {
 
 			if($tags) {
 				$node->nodeValue = $this->replaceTagsWithText($node->nodeValue, $tags, function($idx, $tag) use ($parser){
-					return $parser->callShortcode($tag['open'], $tag['attrs'], $tag['content']);
+					$content = $parser->callShortcode($tag['open'], $tag['attrs'], $tag['content']);
+
+					if ($content === false) {
+						if(ShortcodeParser::$error_behavior == ShortcodeParser::ERROR) {
+							user_error('Unknown shortcode tag '.$tag['open'], E_USER_ERRROR);
+						}
+						else if(ShortcodeParser::$error_behavior == ShortcodeParser::STRIP) {
+							return '';
+						}
+						else {
+							return $tag['text'];
+						}
+					}
+
+					return $content;
 				});
 			}
 		}
diff --git a/tests/parsers/ShortcodeParserTest.php b/tests/parsers/ShortcodeParserTest.php
index 3f571b484..d90b870bc 100644
--- a/tests/parsers/ShortcodeParserTest.php
+++ b/tests/parsers/ShortcodeParserTest.php
@@ -19,18 +19,26 @@ class ShortcodeParserTest extends SapphireTest {
 	 */
 	public function testNotRegisteredShortcode() {
 		ShortcodeParser::$error_behavior = ShortcodeParser::STRIP;
+
 		$this->assertEquals(
 			'',
 			$this->parser->parse('[not_shortcode]')
 		);
 
+		$this->assertEquals(
+			'<img class="">',
+			$this->parser->parse('<img class="[not_shortcode]">')
+		);
+
 		ShortcodeParser::$error_behavior = ShortcodeParser::WARN;
+
 		$this->assertEquals(
 			'<strong class="warning">[not_shortcode]</strong>',
 			$this->parser->parse('[not_shortcode]')
 		);
 		
 		ShortcodeParser::$error_behavior = ShortcodeParser::LEAVE;
+
 		$this->assertEquals('[not_shortcode]',
 			$this->parser->parse('[not_shortcode]'));
 		$this->assertEquals('[not_shortcode /]',
@@ -41,6 +49,11 @@ class ShortcodeParserTest extends SapphireTest {
 			$this->parser->parse('[not_shortcode]a[/not_shortcode]'));
 		$this->assertEquals('[/not_shortcode]',
 			$this->parser->parse('[/not_shortcode]'));
+
+		$this->assertEquals(
+			'<img class="[not_shortcode]">',
+			$this->parser->parse('<img class="[not_shortcode]">')
+		);
 	}
 	
 	public function testSimpleTag() {