diff --git a/pre-upload.py b/pre-upload.py
index 375f695..4c6b783 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -1255,11 +1255,19 @@
 
 def _run_json_check(_project, commit):
   """Checks that all JSON files are syntactically valid."""
-  for f in _filter_files(_get_affected_files(commit), [r'.*\.json']):
+  ret = []
+
+  files = _filter_files(_get_affected_files(commit, relative=True),
+                        [r'.*\.json$'])
+  for f in files:
+    data = _get_file_content(f, commit)
     try:
-      json.load(open(f))
-    except Exception, e:
-      return HookFailure('Invalid JSON in %s: %s' % (f, e))
+      json.loads(data)
+    except Exception as e:
+      ret.append('%s: Invalid JSON: %s' % (f, e))
+
+  if ret:
+    return HookFailure('\n'.join(ret))
 
 
 def _check_manifests(_project, commit):
diff --git a/pre-upload_unittest.py b/pre-upload_unittest.py
index e38fb96..0f36910 100755
--- a/pre-upload_unittest.py
+++ b/pre-upload_unittest.py
@@ -222,6 +222,45 @@
     self.assertFalse(failure)
 
 
+class CheckJson(cros_test_lib.MockTestCase):
+  """Tests for _run_json_check."""
+
+  def setUp(self):
+    self.file_mock = self.PatchObject(pre_upload, '_get_affected_files')
+    self.content_mock = self.PatchObject(pre_upload, '_get_file_content')
+
+  def testNoJson(self):
+    """Nothing should be checked w/no JSON files."""
+    self.file_mock.return_value = [
+        '/foo/bar.txt',
+        '/readme.md',
+    ]
+    ret = pre_upload._run_json_check('PROJECT', 'COMMIT')
+    self.assertIsNone(ret)
+
+  def testValidJson(self):
+    """We should accept valid json files."""
+    self.file_mock.return_value = [
+        '/foo/bar.txt',
+        '/data.json',
+    ]
+    self.content_mock.return_value = '{}'
+    ret = pre_upload._run_json_check('PROJECT', 'COMMIT')
+    self.assertIsNone(ret)
+    self.content_mock.assert_called_once_with('/data.json', 'COMMIT')
+
+  def testInvalidJson(self):
+    """We should reject invalid json files."""
+    self.file_mock.return_value = [
+        '/foo/bar.txt',
+        '/data.json',
+    ]
+    self.content_mock.return_value = '{'
+    ret = pre_upload._run_json_check('PROJECT', 'COMMIT')
+    self.assertIsNotNone(ret)
+    self.content_mock.assert_called_once_with('/data.json', 'COMMIT')
+
+
 class CheckManifests(cros_test_lib.MockTestCase):
   """Tests _check_manifests."""