diff --git a/docs/using/interaction/custom_source.ipynb b/docs/using/interaction/custom_source.ipynb
index 8205681142e..68aaf50a46d 100644
--- a/docs/using/interaction/custom_source.ipynb
+++ b/docs/using/interaction/custom_source.ipynb
@@ -35,7 +35,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Custom packet source class that is derived from BasePacketSource. The method create_packets (which returns ```nus, mus, energies```) has to be defined."
+    "Custom packet source class that is derived from BasePacketSource. The method create_packets (which returns ```radii, nus, mus, energies```) has to be defined."
    ]
   },
   {
@@ -55,7 +55,7 @@
     "        self.rng = np.random.default_rng(seed=seed)\n",
     "        self.truncation_wavelength = truncation_wavelength\n",
     "    \n",
-    "    def create_packets(self, T, no_of_packets, rng,\n",
+    "    def create_packets(self, T, no_of_packets, rng, radius,\n",
     "                       drawing_sample_size=None):\n",
     "        \"\"\"\n",
     "        Packet source that generates a truncated Blackbody source.\n",
@@ -71,6 +71,9 @@
     "            Only wavelengths higher than the truncation wavelength\n",
     "            will be sampled.\n",
     "        \"\"\"\n",
+    "        \n",
+    "        # Makes uniform array of packet radii\n",
+    "        radii = np.ones(no_of_packets) * radius\n",
     "\n",
     "        # Use mus and energies from normal blackbody source.\n",
     "        mus = self.create_zero_limb_darkening_packet_mus(no_of_packets, self.rng)\n",
@@ -100,7 +103,7 @@
     "            additional_nus = additional_nus[mask][:no_of_packets]\n",
     "            nus = np.hstack([nus, additional_nus])[:no_of_packets]\n",
     "            \n",
-    "        return nus, mus, energies"
+    "        return radii, nus, mus, energies"
    ]
   },
   {
@@ -161,7 +164,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.6.7"
+   "version": "3.7.10"
   },
   "nbsphinx": {
    "execute": "always",