diff --git a/src/ground_vehicle.hpp b/src/ground_vehicle.hpp
index 67676fb919..346ee82d13 100644
--- a/src/ground_vehicle.hpp
+++ b/src/ground_vehicle.hpp
@@ -225,7 +225,9 @@ struct GroundVehicle : public SpecializedVehicle<T, Type> {
 			this->z_pos += HasBit(this->gv_flags, GVF_GOINGUP_BIT) ? d : -d;
 		}
 
+#ifdef _DEBUG
 		assert(this->z_pos == GetSlopePixelZ(this->x_pos, this->y_pos));
+#endif
 	}
 
 	/**