diff --git a/notebooks/ch-algorithms/shor.ipynb b/notebooks/ch-algorithms/shor.ipynb
index 7444d8d..e23c8d4 100644
--- a/notebooks/ch-algorithms/shor.ipynb
+++ b/notebooks/ch-algorithms/shor.ipynb
@@ -779,7 +779,7 @@
     "    qc = QuantumCircuit(4+N_COUNT, N_COUNT)\n",
     "    for q in range(N_COUNT):\n",
     "        qc.h(q)     # Initialize counting qubits in state |+>\n",
-    "    qc.x(3+N_COUNT) # And auxiliary register in state |1>\n",
+    "    qc.x(N_COUNT) # And auxiliary register in state |1>\n",
     "    for q in range(N_COUNT): # Do controlled-U operations\n",
     "        qc.append(c_amod15(a, 2**q),\n",
     "                 [q] + [i+N_COUNT for i in range(4)])\n",