diff --git a/src/scgenerator/spectra.py b/src/scgenerator/spectra.py
index 03b375b..df8f417 100644
--- a/src/scgenerator/spectra.py
+++ b/src/scgenerator/spectra.py
@@ -241,9 +241,9 @@ class Propagation(Generic[ParamsOrNone]):
     def load_parameters(self) -> Parameters:
         params = Parameters.from_json(self.io.load_data(PARAMS_FN).decode())
         params.compile_in_place(exhaustive=True, strict=False)
-        for k, v in params.items():
-            if isinstance(v, DataFile):
-                v.io = self.io
+        for _, value in params.items():
+            if isinstance(value, DataFile):
+                value.io = self.io
         return params
 
     def append(self, spectrum: np.ndarray):
@@ -326,7 +326,8 @@ def propagation(
 
     if params is None:
         raise ValueError(
-            f"{file} doesn't exist, but no parameters have been passed to create a new propagation"
+            f"{file} doesn't exist, but no parameters have been "
+            "specified to create a new propagation"
         )
 
     if file is not None:
@@ -383,7 +384,9 @@ def propagation_series(
 
         rest = tqdm(rest)
 
-    spectrum = Spectrum.from_params([prop[:] for prop in rest], parameters)
+    if index is None:
+        index = slice(None)
+    spectrum = Spectrum.from_params([prop[index] for prop in rest], parameters)
     for prop in propagations:
         del prop.parameters
 
diff --git a/tests/test_io_handlers.py b/tests/test_io_handlers.py
index 65198ee..177181e 100644
--- a/tests/test_io_handlers.py
+++ b/tests/test_io_handlers.py
@@ -184,7 +184,7 @@ def test_unique_name():
 def test_propagation_series(tmp_path: Path):
     params = Parameters(**PARAMS)
     with pytest.raises(ValueError):
-        specs, props = propagation_series([])
+        specs, _ = propagation_series([])
 
     flist = [tmp_path / f"prop{i}.zip" for i in range(10)]
     for i, f in enumerate(flist):
@@ -196,5 +196,6 @@ def test_propagation_series(tmp_path: Path):
     assert set(flist) == set(tmp_path.glob("*.zip"))
 
     specs, propagations = propagation_series(flist)
+
     assert specs.shape == (10, params.z_num, params.t_num)
     assert all(prop.parameters.name == f"prop {i}" for i, prop in enumerate(propagations))