diff --git a/segmentation_models_pytorch/encoders/__init__.py b/segmentation_models_pytorch/encoders/__init__.py
index a409a662..842719b0 100644
--- a/segmentation_models_pytorch/encoders/__init__.py
+++ b/segmentation_models_pytorch/encoders/__init__.py
@@ -37,13 +37,23 @@
 
 
 def get_encoder(name, in_channels=3, depth=5, weights=None):
-    Encoder = encoders[name]["encoder"]
+
+    try:
+        Encoder = encoders[name]["encoder"]
+    except KeyError:
+        raise KeyError("Wrong encoder name `{}`, supported encoders: {}".format(name, list(encoders.keys())))
+
     params = encoders[name]["params"]
     params.update(depth=depth)
     encoder = Encoder(**params)
 
     if weights is not None:
-        settings = encoders[name]["pretrained_settings"][weights]
+        try:
+            settings = encoders[name]["pretrained_settings"][weights]
+        except KeyError:
+            raise KeyError("Wrong pretrained weights `{}` for encoder `{}`. Avaliable options are: {}".format(
+                weights, name, list(encoders[name]["pretrained_settings"].keys()),
+            ))
         encoder.load_state_dict(model_zoo.load_url(settings["url"]))
 
     encoder.set_in_channels(in_channels)
diff --git a/segmentation_models_pytorch/pan/model.py b/segmentation_models_pytorch/pan/model.py
index 74a52989..430657b1 100644
--- a/segmentation_models_pytorch/pan/model.py
+++ b/segmentation_models_pytorch/pan/model.py
@@ -44,7 +44,7 @@ class PAN(SegmentationModel):
     def __init__(
             self,
             encoder_name: str = "resnet34",
-            encoder_weights: str = "imagenet",
+            encoder_weights: Optional[str] = "imagenet",
             encoder_dilation: bool = True,
             decoder_channels: int = 32,
             in_channels: int = 3,
diff --git a/segmentation_models_pytorch/unet/model.py b/segmentation_models_pytorch/unet/model.py
index a45753b9..7009cf2d 100644
--- a/segmentation_models_pytorch/unet/model.py
+++ b/segmentation_models_pytorch/unet/model.py
@@ -51,7 +51,7 @@ def __init__(
         self,
         encoder_name: str = "resnet34",
         encoder_depth: int = 5,
-        encoder_weights: str = "imagenet",
+        encoder_weights: Optional[str] = "imagenet",
         decoder_use_batchnorm: bool = True,
         decoder_channels: List[int] = (256, 128, 64, 32, 16),
         decoder_attention_type: Optional[str] = None,
diff --git a/segmentation_models_pytorch/unetplusplus/model.py b/segmentation_models_pytorch/unetplusplus/model.py
index 51a14a5b..3353a317 100644
--- a/segmentation_models_pytorch/unetplusplus/model.py
+++ b/segmentation_models_pytorch/unetplusplus/model.py
@@ -51,7 +51,7 @@ def __init__(
         self,
         encoder_name: str = "resnet34",
         encoder_depth: int = 5,
-        encoder_weights: str = "imagenet",
+        encoder_weights: Optional[str] = "imagenet",
         decoder_use_batchnorm: bool = True,
         decoder_channels: List[int] = (256, 128, 64, 32, 16),
         decoder_attention_type: Optional[str] = None,