diff --git a/src/cpp/FBP45.cpp b/src/cpp/FBP45.cpp
index 79a454f1ecf..ea4c40fa402 100644
--- a/src/cpp/FBP45.cpp
+++ b/src/cpp/FBP45.cpp
@@ -25,7 +25,7 @@ MathSize FuelD1::isfD1(const SpreadInfo& spread,
   calculate_surface_fuel_consumption_c1(const MathSize ffmc) noexcept
 {
   return max(0.0,
-             0.75 + ((ffmc > 84) ? 0.75 : -0.75) * sqrt(1 - exp(-0.23 * (ffmc - 84))));
+             0.75 + ((ffmc > 84) ? 0.75 : -0.75) * sqrt(1 - exp(-0.23 * abs(ffmc - 84))));
 }
 /**
  * \brief Surface Fuel Consumption (SFC) (kg/m^2) [GLC-X-10 eq 9a/9b]
@@ -103,3 +103,4 @@ MathSize FuelD2::calculateRos(const int,
   return (wx.bui().asValue() >= 80) ? rosBasic(isi) : 0.0;
 }
 }
+