diff --git a/source/_components/sensor.fritzbox_callmonitor.markdown b/source/_components/sensor.fritzbox_callmonitor.markdown
index 0666febbf86..867ed606016 100644
--- a/source/_components/sensor.fritzbox_callmonitor.markdown
+++ b/source/_components/sensor.fritzbox_callmonitor.markdown
@@ -26,6 +26,12 @@ $ sudo apt-get install libxml2-dev libxslt-dev \
   python3-setuptools zlib1g-dev build-essential
 ```
 
+If you installed Home Assistant in a virtualenv, also run the following command inside it. 
+Be patient this will take a while.
+```bash
+pip3 install lxml
+```
+
 ## {% linkable_title Setup%}
 
 To activate the call monitor on your Fritz!Box, dial #96\*5\* from any phone connected to it.