diff --git a/dspy/predict/knn.py b/dspy/predict/knn.py
index 68f07b3a63..c88a61b62e 100644
--- a/dspy/predict/knn.py
+++ b/dspy/predict/knn.py
@@ -22,7 +22,7 @@ def __init__(self, k: int, trainset: list[Example], vectorizer: Embedder):
 
             # Create a training dataset with examples
             trainset = [
-                dspy.Example(input="hello", output="world"),
+                dspy.Example(input="hello", output="world").with_inputs("input"),
                 # ... more examples ...
             ]
 
@@ -41,8 +41,7 @@ def __init__(self, k: int, trainset: list[Example], vectorizer: Embedder):
         self.trainset = trainset
         self.embedding = vectorizer
         trainset_casted_to_vectorize = [
-            " | ".join([f"{key}: {value}" for key, value in example.items() if key in example._input_keys])
-            for example in self.trainset
+            " | ".join([f"{key}: {value}" for key, value in example.inputs().items()]) for example in self.trainset
         ]
         self.trainset_vectors = self.embedding(trainset_casted_to_vectorize).astype(np.float32)
 
diff --git a/tests/predict/test_knn.py b/tests/predict/test_knn.py
index 902ecc4f47..1628167741 100644
--- a/tests/predict/test_knn.py
+++ b/tests/predict/test_knn.py
@@ -49,3 +49,20 @@ def test_knn_query_specificity(setup_knn):
     nearest_samples = knn(**query)
     assert len(nearest_samples) == 2, "Incorrect number of nearest samples returned"
     assert "Paris" in [sample.answer for sample in nearest_samples], "Expected Paris to be a nearest sample answer"
+
+
+def test_knn_init_raises_actionable_error_without_with_inputs():
+    """Constructing KNN with examples lacking with_inputs() raises a clear ValueError, not a cryptic TypeError."""
+    trainset = [dspy.Example(question="What is the capital of France?", answer="Paris")]
+    with pytest.raises(ValueError, match="with_inputs"):
+        KNN(k=1, trainset=trainset, vectorizer=dspy.Embedder(DummyVectorizer()))
+
+
+def test_knn_init_mixed_trainset_raises_for_unmarked_example():
+    """A single unmarked example in an otherwise valid trainset still raises the actionable ValueError."""
+    trainset = [
+        dspy.Example(question="What is the largest ocean?", answer="Pacific").with_inputs("question"),
+        dspy.Example(question="What is 2+2?", answer="4"),
+    ]
+    with pytest.raises(ValueError, match="with_inputs"):
+        KNN(k=1, trainset=trainset, vectorizer=dspy.Embedder(DummyVectorizer()))