diff --git a/monai/networks/nets/basic_unet.py b/monai/networks/nets/basic_unet.py
index b26fdcb622..c91335ccba 100644
--- a/monai/networks/nets/basic_unet.py
+++ b/monai/networks/nets/basic_unet.py
@@ -149,12 +149,12 @@ def __init__(
         self.convs = TwoConv(spatial_dims, cat_chns + up_chns, out_chns, act, norm, bias, dropout)
         self.is_pad = is_pad
 
-    def forward(self, x: torch.Tensor, x_e: torch.Tensor | None):
+    def forward(self, x: torch.Tensor, x_e: torch.Tensor):
         """
 
         Args:
             x: features to be upsampled.
-            x_e: features from the encoder.
+            x_e: optional features from the encoder, if None, this branch is not in use.
         """
         x_0 = self.upsample(x)
 
diff --git a/monai/networks/nets/flexible_unet.py b/monai/networks/nets/flexible_unet.py
index a880cafdc3..ac2124b5f9 100644
--- a/monai/networks/nets/flexible_unet.py
+++ b/monai/networks/nets/flexible_unet.py
@@ -232,6 +232,7 @@ def __init__(
         dropout: float | tuple = 0.0,
         decoder_bias: bool = False,
         upsample: str = "nontrainable",
+        pre_conv: str = "default",
         interp_mode: str = "nearest",
         is_pad: bool = True,
     ) -> None:
@@ -262,6 +263,8 @@ def __init__(
             decoder_bias: whether to have a bias term in decoder's convolution blocks.
             upsample: upsampling mode, available options are``"deconv"``, ``"pixelshuffle"``,
                 ``"nontrainable"``.
+            pre_conv:a conv block applied before upsampling. Only used in the "nontrainable" or
+                "pixelshuffle" mode, default to `default`.
             interp_mode: {``"nearest"``, ``"linear"``, ``"bilinear"``, ``"bicubic"``, ``"trilinear"``}
                 Only used in the "nontrainable" mode.
             is_pad: whether to pad upsampling features to fit features from encoder. Default to True.
@@ -309,7 +312,7 @@ def __init__(
             bias=decoder_bias,
             upsample=upsample,
             interp_mode=interp_mode,
-            pre_conv="default",
+            pre_conv=pre_conv,
             align_corners=None,
             is_pad=is_pad,
         )