diff --git a/monitor/fan.hpp b/monitor/fan.hpp
index a932db2..713a1a1 100644
--- a/monitor/fan.hpp
+++ b/monitor/fan.hpp
@@ -196,7 +196,7 @@
     /**
      * @brief Returns the number sensors that are nonfunctional
      */
-    size_t countNonFunctionalSensors();
+    size_t countNonFunctionalSensors() const;
 
     /**
      * @brief Updates the Functional property in the inventory