Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/vendor/symfony/yaml/Parser.php

@@ -234,7 +234,7 @@ class Parser
                 }
 
                 // 1-liner optionally followed by newline(s)
-                if ($this->lines[0] === trim($value)) {
+                if (is_string($value) && $this->lines[0] === trim($value)) {
                     try {
                         $value = Inline::parse($this->lines[0], $exceptionOnInvalidType, $objectSupport, $objectForMap, $this->refs);
                     } catch (ParseException $e) {
@@ -356,7 +356,7 @@ class Parser
             return;
         }
 
-        if ($inSequence && $oldLineIndentation === $newIndent && '-' === $data[0][0]) {
+        if ($inSequence && $oldLineIndentation === $newIndent && isset($data[0][0]) && '-' === $data[0][0]) {
             // the previous line contained a dash but no item content, this line is a sequence item with the same indentation
             // and therefore no nested list or mapping
             $this->moveToPreviousLine();

core/vendor/symfony/yaml/Tests/ParserTest.php

@@ -551,6 +551,21 @@ EOF
         );
     }
 
+    /**
+     * @expectedException \Symfony\Component\Yaml\Exception\ParseException
+     * @expectedExceptionMessage missing colon
+     */
+    public function testScalarInSequence()
+    {
+        Yaml::parse(<<<EOF
+foo:
+    - bar
+"missing colon"
+    foo: bar
+EOF
+        );
+    }
+
     /**
      * > It is an error for two equal keys to appear in the same mapping node.
      * > In such a case the YAML processor may continue, ignoring the second