diff --git a/fdroidserver/index.py b/fdroidserver/index.py
index 0beba4ef..873f401e 100644
--- a/fdroidserver/index.py
+++ b/fdroidserver/index.py
@@ -1516,6 +1516,7 @@ def add_mirrors_to_repodict(repo_section, repodict):
     repodict['mirrors'] = []
     canonical_url = repodict['address']
     found_primary = False
+    errors = 0
     for mirror in mirrors:
         if canonical_url == mirror['url']:
             found_primary = True
@@ -1524,9 +1525,19 @@ def add_mirrors_to_repodict(repo_section, repodict):
             for k in sorted(mirror.keys()):
                 sortedmirror[k] = mirror[k]
             repodict['mirrors'].insert(0, sortedmirror)
+        elif mirror.get('isPrimary'):
+            errors += 1
+            logging.error(
+                _('Mirror config for {url} contains "isPrimary" key!').format(
+                    url=mirror['url']
+                )
+            )
         else:
             repodict['mirrors'].append(mirror)
 
+    if errors:
+        raise FDroidException(_('"isPrimary" key should not be added to mirrors!'))
+
     if repodict['mirrors'] and not found_primary:
         repodict['mirrors'].insert(0, {'isPrimary': True, 'url': repodict['address']})
 
diff --git a/tests/test_index.py b/tests/test_index.py
index 059386fb..3e078eef 100755
--- a/tests/test_index.py
+++ b/tests/test_index.py
@@ -814,6 +814,19 @@ class IndexTest(unittest.TestCase):
         with self.assertRaises(fdroidserver.exception.FDroidException):
             index.add_mirrors_to_repodict('repo', repodict)
 
+    def test_erroneous_isPrimary_in_mirrors_config(self):
+        """There can be only one primary mirror aka canonical URL"""
+        common.config = {
+            'repo_url': 'http://one/fdroid',
+            'mirrors': [
+                {'url': 'http://one/fdroid', 'countryCode': 'SA'},
+                {'url': 'http://two/fdroid', 'isPrimary': True},
+            ],
+        }
+        repodict = {'address': common.config['repo_url']}
+        with self.assertRaises(fdroidserver.exception.FDroidException):
+            index.add_mirrors_to_repodict('repo', repodict)
+
 
 class AltstoreIndexTest(unittest.TestCase):
     def test_make_altstore(self):