diff --git a/dive.c b/dive.c
index 4d4cc3e1d..5732c0f83 100644
--- a/dive.c
+++ b/dive.c
@@ -1683,6 +1683,8 @@ extern void fill_pressures(struct gas_pressures *pressures, const double amb_pre
 	} else {
 		if (divemode == PSCR) { /* The steady state approximation should be good enough */
 			pressures->o2 = get_o2(mix) / 1000.0 * amb_pressure - (1.0 - get_o2(mix) / 1000.0) * prefs.o2consumption / (prefs.bottomsac * prefs.pscr_ratio / 1000.0);
+			if (pressures->o2 < 0) // He's dead, Jim.
+				pressures->o2 = 0;
 			if (get_o2(mix) != 1000) {
 				pressures->he = (amb_pressure - pressures->o2) * get_he(mix) / (1000.0 - get_o2(mix));
 				pressures->n2 = (amb_pressure - pressures->o2) * (1000 - get_o2(mix) - get_he(mix)) / (1000.0 - get_o2(mix));