diff --git a/src/xrt/auxiliary/math/m_relation_history.cpp b/src/xrt/auxiliary/math/m_relation_history.cpp
index 72f1f7e31..0bc21b290 100644
--- a/src/xrt/auxiliary/math/m_relation_history.cpp
+++ b/src/xrt/auxiliary/math/m_relation_history.cpp
@@ -236,8 +236,9 @@ m_relation_history_get_latest(struct m_relation_history *rh,
 }
 
 uint32_t
-m_relation_history_get_size(const struct m_relation_history *rh)
+m_relation_history_get_size(struct m_relation_history *rh)
 {
+	std::unique_lock<os::Mutex> lock(rh->mutex);
 	return (uint32_t)rh->impl.size();
 }
 
diff --git a/src/xrt/auxiliary/math/m_relation_history.h b/src/xrt/auxiliary/math/m_relation_history.h
index 3e6e84def..897c2765c 100644
--- a/src/xrt/auxiliary/math/m_relation_history.h
+++ b/src/xrt/auxiliary/math/m_relation_history.h
@@ -109,7 +109,7 @@ m_relation_history_get_latest(struct m_relation_history *rh,
  * @public @memberof m_relation_history
  */
 uint32_t
-m_relation_history_get_size(const struct m_relation_history *rh);
+m_relation_history_get_size(struct m_relation_history *rh);
 
 /*!
  * Clears the history from all of the items.
@@ -199,7 +199,7 @@ public:
 	 * @copydoc m_relation_history_get_size
 	 */
 	size_t
-	size() const noexcept
+	size() noexcept
 	{
 		return m_relation_history_get_size(mPtr);
 	}