diff --git a/src/Environment/atmosphere.cxx b/src/Environment/atmosphere.cxx
index dc823292f..a6720c241 100644
--- a/src/Environment/atmosphere.cxx
+++ b/src/Environment/atmosphere.cxx
@@ -20,7 +20,8 @@ ISA_layer(4,  47000,  154199,  110.906,   0.0327506, 270.65,  -2.50,       0,
 ISA_layer(5,  51000,  167322,  66.9389,   0.0197670, 270.65,  -2.50,  0.0028,   0.0008534),
 ISA_layer(6,  71000,  232939,  3.95642,  0.00116833, 214.65, -58.50,  0.0020,   0.0006096),
 ISA_layer(7,  80000,  262467,  0.88628, 0.000261718, 196.65, -76.50,  0.0,      0.0),
-ISA_layer(8,  1.0e9,  3.28e9,  0.00001, 3.0e-9,        2.73, -270.4,  0.0,      0.0),
+// The last layer MUST have -1.0 for its 'lapse' field
+ISA_layer(8,  1.0e9,  3.28e9,  0.00001, 3.0e-9,        2.73, -270.4,  -1.0)
 };
 
 // Pressure within a layer, as a function of height.