diff --git a/lambeq/backend/pennylane.py b/lambeq/backend/pennylane.py
index f4431f8c..0ec36d7a 100644
--- a/lambeq/backend/pennylane.py
+++ b/lambeq/backend/pennylane.py
@@ -49,7 +49,7 @@
 import sympy
 import torch
 
-from lambeq.backend.quantum import Scalar
+from lambeq.backend.quantum import Scalar, Measure
 
 if TYPE_CHECKING:
     from lambeq.backend.quantum import Diagram
@@ -217,6 +217,10 @@ def to_pennylane(lambeq_circuit: Diagram, probabilities=False,
 
     """
 
+    if any(isinstance(box, Measure) for box in lambeq_circuit.boxes):
+        raise ValueError("Only pure circuits, or circuits with discards"
+                          " are currently supported.")
+
     tk_circ = lambeq_circuit.to_tk()
     op_list, params_list, wires_list, symbols_set = (
         extract_ops_from_tk(tk_circ)