diff --git a/wpilibc/Athena/src/Ultrasonic.cpp b/wpilibc/Athena/src/Ultrasonic.cpp
index f6c99d86db..183e1ce1f9 100644
--- a/wpilibc/Athena/src/Ultrasonic.cpp
+++ b/wpilibc/Athena/src/Ultrasonic.cpp
@@ -25,6 +25,7 @@ constexpr double Ultrasonic::kSpeedOfSoundInchesPerSec;
 Task Ultrasonic::m_task;
 // automatic round robin mode
 std::atomic<bool> Ultrasonic::m_automaticEnabled{false};
+std::set<Ultrasonic*> Ultrasonic::m_sensors;
 
 /**
  * Background task that goes through the list of ultrasonic sensors and pings