diff --git a/deco.c b/deco.c
index 6e73c50cb..023a20f72 100644
--- a/deco.c
+++ b/deco.c
@@ -126,7 +126,7 @@ static double tissue_tolerance_calc(const struct dive *dive)
 }
 
 /* add period_in_seconds at the given pressure and gas to the deco calculation */
-double add_segment(double pressure, struct gasmix *gasmix, int period_in_seconds, int ccpo2, const struct dive *dive)
+double add_segment(double pressure, const struct gasmix *gasmix, int period_in_seconds, int ccpo2, const struct dive *dive)
 {
 	int ci;
 	int fo2 = gasmix->o2.permille ? gasmix->o2.permille : O2_IN_AIR;
diff --git a/dive.h b/dive.h
index 54c4fd470..fcbaca713 100644
--- a/dive.h
+++ b/dive.h
@@ -598,7 +598,7 @@ extern void subsurface_command_line_exit(gint *, gchar ***);
 
 #define FRACTION(n,x) ((unsigned)(n)/(x)),((unsigned)(n)%(x))
 
-extern double add_segment(double pressure, struct gasmix *gasmix, int period_in_seconds, int setpoint, const struct dive *dive);
+extern double add_segment(double pressure, const struct gasmix *gasmix, int period_in_seconds, int setpoint, const struct dive *dive);
 extern void clear_deco(double surface_pressure);
 extern void dump_tissues(void);
 extern unsigned int deco_allowed_depth(double tissues_tolerance, double surface_pressure, struct dive *dive, gboolean smooth);