diff --git a/braintools/file/msg_checkpoint.py b/braintools/file/msg_checkpoint.py
index f0c2fe6..97f59b8 100644
--- a/braintools/file/msg_checkpoint.py
+++ b/braintools/file/msg_checkpoint.py
@@ -835,7 +835,7 @@ def msgpack_save(
     filename: str
       str or pathlib-like path to store checkpoint files in.
     target: Any
-      serializable flax object, usually a flax optimizer.
+      serializable object.
     overwrite: bool
       overwrite existing checkpoint files if a checkpoint at the
       current or a later step already exits (default: False).
@@ -864,6 +864,9 @@ def msgpack_save(
         os.makedirs(os.path.dirname(filename), exist_ok=True)
     if not overwrite and os.path.exists(filename):
         raise InvalidCheckpointPath(filename)
+
+    if isinstance(target, bst.util.FlattedDict):
+        target = target.to_nest()
     target = to_bytes(target)
 
     # Save the files via I/O sync or async.
@@ -943,6 +946,9 @@ def msgpack_load(
     sys.stdout.flush()
     file_size = os.path.getsize(filename)
 
+    if isinstance(target, bst.util.FlattedDict):
+        target = target.to_nest()
+
     with open(filename, 'rb') as fp:
         if parallel and fp.seekable():
             buf_size = 128 << 20  # 128M buffer.
diff --git a/braintools/metric/_firings.py b/braintools/metric/_firings.py
index ae6990b..fdaeb15 100644
--- a/braintools/metric/_firings.py
+++ b/braintools/metric/_firings.py
@@ -18,7 +18,7 @@
 from typing import Union
 
 import brainstate as bst
-import brainunit as bu
+import brainunit as u
 import jax.numpy as jnp
 import numpy as onp
 
@@ -56,8 +56,8 @@ def raster_plot(
 
 def firing_rate(
     spikes: bst.typing.ArrayLike,
-    width: Union[float, bu.Quantity],
-    dt: Union[float, bu.Quantity] = None
+    width: Union[float, u.Quantity],
+    dt: Union[float, u.Quantity] = None
 ):
   r"""Calculate the mean firing rate over in a neuron group.
 
@@ -86,5 +86,7 @@ def firing_rate(
   """
   dt = bst.environ.get_dt() if (dt is None) else dt
   width1 = int(width / 2 / dt) * 2 + 1
-  window = jnp.ones(width1) * 1000 / width
+  window = u.math.ones(width1) / width
+  if isinstance(window, u.Quantity):
+    window = window.to_decimal(u.Hz)
   return jnp.convolve(jnp.mean(spikes, axis=1), window, mode='same')
diff --git a/braintools/visualize/style.py b/braintools/visualize/style.py
index b880fea..5b7da57 100644
--- a/braintools/visualize/style.py
+++ b/braintools/visualize/style.py
@@ -13,10 +13,10 @@
 # limitations under the License.
 # ==============================================================================
 
-import matplotlib.pyplot as plt
-from matplotlib import RcParams
 
 try:
+  import matplotlib.pyplot as plt
+  from matplotlib import RcParams
   import scienceplots  # noqa: F401
 
   def exclude(rc: RcParams, keys: list):