diff --git a/cellpose/gui/gui.py b/cellpose/gui/gui.py
index 99c7fcd7..a61cd7a7 100644
--- a/cellpose/gui/gui.py
+++ b/cellpose/gui/gui.py
@@ -1770,7 +1770,7 @@ def initialize_model(self, model_name=None, custom=False):
         if model_name is None or custom:
             self.get_model_path(custom=custom)
             if not os.path.exists(self.current_model_path):
-                raise ValueError("need to specify model (use dropdown)")
+                raise ValueError("Model file not found: need to specify model (use dropdown)")
 
         if model_name is None or not isinstance(model_name, str):
             self.model = models.CellposeModel(gpu=self.useGPU.isChecked(),
@@ -1867,7 +1867,7 @@ def compute_cprob(self):
                 self.logger.error("Flows don't exist, try running model again.")
                 return
             
-            maski = dynamics.resize_and_compute_masks(
+            maski = dynamics.compute_masks_and_clean(
                 dP=dP,
                 cellprob=cellprob,
                 niter=niter,
diff --git a/cellpose/vit_sam.py b/cellpose/vit_sam.py
index 7e82bd18..7d93378a 100644
--- a/cellpose/vit_sam.py
+++ b/cellpose/vit_sam.py
@@ -81,9 +81,15 @@ def forward(self, x):
            
         return x1, torch.zeros((x.shape[0], 256), device=x.device)
     
-    def load_model(self, PATH, device, strict = False):        
+    def load_model(self, PATH, device, strict = False):
         state_dict = torch.load(PATH, map_location = device, weights_only=True)
         keys = [k for k in state_dict.keys()]
+
+        # loudly fail on attempt to load not cp4 model: 
+        w2_data = state_dict.get('W2', None)
+        if w2_data == None:
+            raise ValueError('This model does not appear to be a CP4 model. CP3 models are not compatible with CP4.')
+
         if keys[0][:7] == "module.":
             from collections import OrderedDict
             new_state_dict = OrderedDict()
diff --git a/tests/test_import.py b/tests/test_import.py
index 6407d116..2f4c709f 100644
--- a/tests/test_import.py
+++ b/tests/test_import.py
@@ -1,3 +1,6 @@
+import pytest
+
+
 def test_cellpose_imports_without_error():
     import cellpose
     from cellpose import models, core
@@ -28,3 +31,18 @@ def itest_model_dir():
     model = models.CellposeModel(pretrained_model='cpsam')
     masks = model.eval(np.random.randn(256, 256))[0]
     assert masks.shape == (256, 256)
+
+
+def test_load_cp3_fail():
+    from cellpose.models import CellposeModel, MODEL_DIR
+    from cellpose import utils
+
+    cyto3_model_path = (MODEL_DIR / 'cyto3').absolute()
+
+    if not cyto3_model_path.exists():
+        url = 'https://www.cellpose.org/models/cyto3'
+        utils.download_url_to_file(url, cyto3_model_path, progress=False)
+
+    with pytest.raises(ValueError):
+        # using `pretrained_model=cyto3` just loads the cpsam model unless the path is given
+        model = CellposeModel(pretrained_model=str(cyto3_model_path))
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