Index: src/cmd/movable.cpp
--- src/cmd/movable.cpp.orig
+++ src/cmd/movable.cpp
@@ -685,7 +685,7 @@ Vector Movable::ClampThrust(const Vector &amt1, bool a
                         * simulation_atom_var * Res.Magnitude()
                         * FMEC_exit_vel_inverse / Lithium6constant;
 #ifndef __APPLE__
-        if (ISNAN(unit->fuel)) {
+        if (std::isnan(unit->fuel)) {
             VS_LOG(error, "Fuel is NAN A");
             unit->fuel = 0;
         }
@@ -696,7 +696,7 @@ Vector Movable::ClampThrust(const Vector &amt1, bool a
         //HACK this forces the reaction to be Li-6+Li-6 fusion with efficiency governed by the getFuelUsage function
         unit->fuel -= unit->getFuelUsage(false) * simulation_atom_var * Res.Magnitude() * FMEC_exit_vel_inverse / Lithium6constant;
 #ifndef __APPLE__
-        if (ISNAN(unit->fuel)) {
+        if (std::isnan(unit->fuel)) {
             VS_LOG(error, "Fuel is NAN B");
             unit->fuel = 0;
         }
