diff --git a/mhcflurry/class1_neural_network.py b/mhcflurry/class1_neural_network.py
index b6c4945dd6e689cbc5e0be7f826bd11ce1ec7294..b185fdba5c4cff6c9f466bc2bc801ec950ac2c53 100644
--- a/mhcflurry/class1_neural_network.py
+++ b/mhcflurry/class1_neural_network.py
@@ -955,4 +955,6 @@ class Class1NeuralNetwork(object):
         if existing.shape == allele_representations.shape:
             layer.set_weights([allele_representations])
         else:
-            raise NotImplementedError("Network surgery required")
+            raise NotImplementedError(
+                "Network surgery required: %s != %s" % (
+                    str(existing), str(allele_representations.shape)))