diff --git a/pmb/helpers/repo.py b/pmb/helpers/repo.py
index a27de364..5738a9fb 100644
--- a/pmb/helpers/repo.py
+++ b/pmb/helpers/repo.py
@@ -53,8 +53,16 @@ def diff(args, files_a, files_b=None):
     ret = []
     for arch in files_b.keys():
         for file, timestamp in files_b[arch].items():
-            if (arch not in files_a or file not in files_a[arch] or
-                    timestamp is not files_a[arch][file]):
+            add = False
+            if arch not in files_a:
+                add = True
+            elif file not in files_a[arch]:
+                add = True
+            elif timestamp != files_a[arch][file]:
+                add = True
+            if add:
                 ret.append(arch + "/" + file)
 
+
+
     return sorted(ret)
diff --git a/test/repo.py b/test/repo.py
new file mode 100644
index 00000000..c570e29b
--- /dev/null
+++ b/test/repo.py
@@ -0,0 +1,87 @@
+"""
+Copyright 2017 Oliver Smith
+
+This file is part of pmbootstrap.
+
+pmbootstrap is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+pmbootstrap is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with pmbootstrap.  If not, see <http://www.gnu.org/licenses/>.
+"""
+import os
+import sys
+import pytest
+import types
+import pathlib
+import time
+
+# Import from parent directory
+pmb_src = os.path.abspath(os.path.join(os.path.dirname(__file__) + "/.."))
+sys.path.append(pmb_src)
+import pmb.helpers.repo
+
+
+@pytest.fixture
+def args(request, tmpdir):
+    args = types.SimpleNamespace()
+    args.work = str(tmpdir)
+    return args
+
+def clear_timestamps_from_files(files):
+    """
+    Replace all last modified timestamps from pmb.helpers.repo.files() with
+    None. The files-parameter gets changed in place.
+    """
+    for arch in files.keys():
+        for file in files[arch].keys():
+            files[arch][file] = None
+
+
+def test_files_empty(args):
+    os.mkdir(args.work + "/packages")
+    assert pmb.helpers.repo.files(args) == {}
+
+
+def test_files_not_empty(args):
+    pkgs = args.work + "/packages"
+    for dir in ["", "armhf", "x86_64"]:
+        os.mkdir(pkgs + "/" + dir)
+    open(pkgs+"/x86_64/test", "a").close()
+    files = pmb.helpers.repo.files(args)
+    clear_timestamps_from_files(files)
+    assert files == {"armhf": {}, "x86_64": {"test": None}}
+
+
+def test_files_diff(args):
+    # Create x86_64/test, x86_64/test2
+    pkgs = args.work + "/packages"
+    for dir in ["", "x86_64"]:
+        os.mkdir(pkgs + "/" + dir)
+    for file in ["x86_64/test", "x86_64/test2"]:
+        open(pkgs + "/" + file, "a").close()
+
+    # First snapshot
+    first = pmb.helpers.repo.files(args)
+
+    # Change: x86_64/test (set the lastmod timestamp 5 seconds in the future)
+    mtime_old = os.path.getmtime(pkgs + "/x86_64/test")
+    time_new = time.time() + 5
+    os.utime(pkgs + "/x86_64/test", (time_new, time_new))
+    mtime_new = os.path.getmtime(pkgs + "/x86_64/test")
+    assert mtime_old != mtime_new
+
+    # Create: aarch64/test3, x86_64/test4
+    os.mkdir(pkgs + "/aarch64")
+    open(pkgs + "/aarch64/test3", "a").close()
+    open(pkgs + "/x86_64/test4", "a").close()
+
+    diff = pmb.helpers.repo.diff(args, first)
+    assert diff == ["aarch64/test3", "x86_64/test", "x86_64/test4"]