diff --git a/fdroidserver/metadata.py b/fdroidserver/metadata.py
index a945b589..a4c6bf00 100644
--- a/fdroidserver/metadata.py
+++ b/fdroidserver/metadata.py
@@ -3,6 +3,7 @@
 # metadata.py - part of the FDroid server tools
 # Copyright (C) 2013, Ciaran Gultnieks, ciaran@ciarang.com
 # Copyright (C) 2013-2014 Daniel Martí <mvdan@mvdan.cc>
+# Copyright (C) 2017-2018 Michael Pöhn <michael.poehn@fsfe.org>
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU Affero General Public License as published by
@@ -1078,11 +1079,16 @@ def parse_yaml_metadata(mf, app):
         if field not in yaml_app_fields:
             warn_or_exception(_('Unrecognised app field: {fieldname}')
                               .format(fieldname=field))
-    if 'Builds' not in yamldata.keys():
+    if not yamldata.get('Builds', None):
         warn_or_exception(_('Missing app field: {fieldname}')
                           .format(fieldname='Builds'))
-    for build in yamldata['Builds']:
+    for build in yamldata.get('Builds', []):
+        # put all build flag keywords into a set to avoid
+        # excessive looping action
+        build_flag_set = set()
         for build_flag in build.keys():
+            build_flag_set.add(build_flag)
+        for build_flag in build_flag_set:
             if build_flag not in build_flags:
                 warn_or_exception(_('Unrecognised build flag: {build_flag}')
                                   .format(build_flag=build_flag))
diff --git a/tests/metadata.TestCase b/tests/metadata.TestCase
index 247f6a89..c2cf871a 100755
--- a/tests/metadata.TestCase
+++ b/tests/metadata.TestCase
@@ -2,6 +2,7 @@
 
 # http://www.drdobbs.com/testing/unit-testing-with-python/240165163
 
+import io
 import glob
 import inspect
 import logging
@@ -13,6 +14,8 @@ import sys
 import unittest
 import yaml
 import tempfile
+import textwrap
+from unittest import mock
 
 localmodule = os.path.realpath(
     os.path.join(os.path.dirname(inspect.getfile(inspect.currentframe())), '..'))
@@ -22,6 +25,7 @@ if localmodule not in sys.path:
 
 import fdroidserver.common
 import fdroidserver.metadata
+from fdroidserver.exception import MetaDataException
 
 
 class MetadataTest(unittest.TestCase):
@@ -139,6 +143,26 @@ class MetadataTest(unittest.TestCase):
             allappids.append(appid)
         self.assertEqual(randomlist, allappids)
 
+    def test_parse_yaml_metadata_unknown_app_field(self):
+        mf = io.StringIO(textwrap.dedent("""\
+            AutoName: F-Droid
+            RepoType: git
+            Builds: []
+            bad: value"""))
+        with mock.patch('fdroidserver.metadata.warnings_action', 'error'):
+            with self.assertRaises(MetaDataException):
+                fdroidserver.metadata.parse_yaml_metadata(mf, {})
+
+    def test_parse_yaml_metadata_unknown_build_flag(self):
+        mf = io.StringIO(textwrap.dedent("""\
+            AutoName: F-Droid
+            RepoType: git
+            Builds:
+              - bad: value"""))
+        with mock.patch('fdroidserver.metadata.warnings_action', 'error'):
+            with self.assertRaises(MetaDataException):
+                fdroidserver.metadata.parse_yaml_metadata(mf, {})
+
 
 if __name__ == "__main__":
     parser = optparse.OptionParser()