diff --git a/cibuildwheel/util.py b/cibuildwheel/util.py
index aade60b2..5575f7cc 100644
--- a/cibuildwheel/util.py
+++ b/cibuildwheel/util.py
@@ -86,7 +86,7 @@ def download(url, dest):
 class DependencyConstraints(object):
     def __init__(self, base_file_path):
         assert os.path.exists(base_file_path)
-        self.base_file_path = base_file_path
+        self.base_file_path = os.path.abspath(base_file_path)
 
     @classmethod
     def with_defaults(cls):