diff --git a/src/docs/part2/adding_detectors.md b/src/docs/part2/adding_detectors.md
index 2fa7f4f69bf3cddd4380983a4db0b8aeefc65a0a..f398ce18f3870e405c442f4227cc2e347cecd33a 100644
--- a/src/docs/part2/adding_detectors.md
+++ b/src/docs/part2/adding_detectors.md
@@ -135,7 +135,7 @@ attribute defined. We will return to this later.
 ####  Critical parts of build_detector
 
 
-We will now look at the function in `src/TrapEndcapTracker_geo.cpp`.
+We will now look at the function in `src/GEMTrackerDisc_geo.cpp`.
 Note that **this is not a good example to copy**, rather it is a simple example to walk through.
 
 ```cpp
@@ -192,6 +192,7 @@ static Ref_t create_detector(Detector& lcdd, xml_h e, SensitiveDetector sens)
   assembly->GetShape()->ComputeBBox() ;
   return sdet;
 }
+DECLARE_DETELEMENT(my_GEMTracker, create_detector)
 ```
 Here is the XML again:
 ```xml