diff --git a/libraries/AP_RSSI/AP_RSSI.cpp b/libraries/AP_RSSI/AP_RSSI.cpp
index dfd83f8b5d..742eab037d 100644
--- a/libraries/AP_RSSI/AP_RSSI.cpp
+++ b/libraries/AP_RSSI/AP_RSSI.cpp
@@ -183,7 +183,7 @@ float AP_RSSI::scale_and_constrain_float_rssi(float current_rssi_value, float lo
     if (range_is_inverted)
     {
         // Swap values so we can treat them as low->high uniformly in the code that follows
-        current_rssi_value = high_rssi_range + abs(current_rssi_value - low_rssi_range);
+        current_rssi_value = high_rssi_range + fabsf(current_rssi_value - low_rssi_range);
         std::swap(low_rssi_range, high_rssi_range);        
     }