diff --git a/Chapter1_Introduction/Ch1_Introduction_PyMC3.ipynb b/Chapter1_Introduction/Ch1_Introduction_PyMC3.ipynb
index 906a485e..4dc61fbb 100644
--- a/Chapter1_Introduction/Ch1_Introduction_PyMC3.ipynb
+++ b/Chapter1_Introduction/Ch1_Introduction_PyMC3.ipynb
@@ -658,7 +658,7 @@
     "    lambda_1 = pm.Exponential(\"lambda_1\", alpha)\n",
     "    lambda_2 = pm.Exponential(\"lambda_2\", alpha)\n",
     "    \n",
-    "    tau = pm.DiscreteUniform(\"tau\", lower=0, upper=n_count_data)"
+    "    tau = pm.DiscreteUniform(\"tau\", lower=0, upper=n_count_data - 1)"
    ]
   },
   {