diff --git a/py/rvspecfit/serializer.py b/py/rvspecfit/serializer.py
index a752382..2348176 100644
--- a/py/rvspecfit/serializer.py
+++ b/py/rvspecfit/serializer.py
@@ -150,7 +150,7 @@ def load_dict_from_hdf5(filename):
     Loads the dictionary from an HDF5 file.
     """
     if not os.path.exists(filename):
-        raise RuntimeError('Filename {filename} does not exist')
+        raise RuntimeError(f'Filename {filename} does not exist')
     with h5py.File(filename, 'r') as h5file:
         version = h5file.attrs.get('version', None)
         if version != CURRENT_VERSION: