diff --git a/buildman/component/buildcomponent.py b/buildman/component/buildcomponent.py
index 52ac7abfb..e1fa37cf2 100644
--- a/buildman/component/buildcomponent.py
+++ b/buildman/component/buildcomponent.py
@@ -18,6 +18,7 @@ from buildman.jobutil.workererror import WorkerError
 from data import model
 from data.database import BUILD_PHASE
 from data.model import InvalidRepositoryBuildException
+from util import slash_join
 
 HEARTBEAT_DELTA = datetime.timedelta(seconds=60)
 BUILD_HEARTBEAT_DELAY = datetime.timedelta(seconds=30)
@@ -180,9 +181,10 @@ class BuildComponent(BaseComponent):
     context = build_config.get('context', '')
     if not (dockerfile_path == '' or context == ''):
       # This should not happen and can be removed when we centralize validating build_config
-      if ".." in os.path.relpath(dockerfile_path, context):
+      dockerfile_abspath = slash_join('', dockerfile_path)
+      if ".." in os.path.relpath(dockerfile_abspath, context):
         return os.path.split(dockerfile_path)
-      dockerfile_path = os.path.relpath(dockerfile_path, context)
+      dockerfile_path = os.path.relpath(dockerfile_abspath, context)
 
     return context, dockerfile_path
 
diff --git a/buildman/component/test/test_buildcomponent.py b/buildman/component/test/test_buildcomponent.py
index a434dd092..c4e026916 100644
--- a/buildman/component/test/test_buildcomponent.py
+++ b/buildman/component/test/test_buildcomponent.py
@@ -23,11 +23,12 @@ def test_path_is_dockerfile(input, expected_path, expected_file):
   assert actual_file == expected_file
 
 @pytest.mark.parametrize('build_config,context,dockerfile_path', [
-  ({}, "", ""),
-  ({'build_subdir': "/builddir/Dockerfile"}, "", "/builddir/Dockerfile"),
-  ({'context': "/builddir"}, "/builddir", ""),
-  ({'context': "/builddir", 'build_subdir': "/builddir/Dockerfile"}, "/builddir", "Dockerfile"),
-  ({'context': "/some_other_dir/Dockerfile", 'build_subdir': "/builddir/Dockerfile"}, "/builddir", "Dockerfile"),
+  ({}, '', ''),
+  ({'build_subdir': '/builddir/Dockerfile'}, '', '/builddir/Dockerfile'),
+  ({'context': '/builddir'}, '/builddir', ''),
+  ({'context': '/builddir', 'build_subdir': '/builddir/Dockerfile'}, '/builddir', 'Dockerfile'),
+  ({'context': '/some_other_dir/Dockerfile', 'build_subdir': '/builddir/Dockerfile'}, '/builddir', 'Dockerfile'),
+  ({'context': '/', 'build_subdir':'Dockerfile'}, '/', 'Dockerfile')
 ])
 def test_extract_dockerfile_args(build_config, context, dockerfile_path):
   actual_context, actual_dockerfile_path = BuildComponent.extract_dockerfile_args(build_config)
diff --git a/util/__init__.py b/util/__init__.py
index c45aad78f..f4744897a 100644
--- a/util/__init__.py
+++ b/util/__init__.py
@@ -10,8 +10,8 @@ def slash_join(*args):
   are not deduplicated.
   """
   def rmslash(path):
-    path = path[1:] if path[0] == '/' else path
-    path = path[:-1] if path[-1] == '/' else path
+    path = path[1:] if len(path) > 0 and path[0] == '/' else path
+    path = path[:-1] if len(path) > 0 and path[-1] == '/' else path
     return path
 
   args = [rmslash(path) for path in args]