diff --git a/libact/labelers/ideal_labeler.py b/libact/labelers/ideal_labeler.py
index dc89efc..a45d0b3 100644
--- a/libact/labelers/ideal_labeler.py
+++ b/libact/labelers/ideal_labeler.py
@@ -26,4 +26,6 @@ def __init__(self, dataset, **kwargs):
         self.y = y
 
     def label(self, feature):
+        if np.count_nonzero([np.array_equal(x, feature) for x in self.X])==0
+            raise ValueError("No matching sample found in the dataset.")
         return self.y[np.where([np.array_equal(x, feature) for x in self.X])[0]]