diff --git a/rdcon/dcon.f b/rdcon/dcon.f
index 9874298b..287b7206 100644
--- a/rdcon/dcon.f
+++ b/rdcon/dcon.f
@@ -222,6 +222,7 @@ SUBROUTINE dcon_run
 
       LOGICAL :: cyl_flag=.FALSE.,regrid_flag=.FALSE.,verbose=.TRUE.
       INTEGER :: mmin,ipsi
+      REAL(r8) :: zeff0
       COMPLEX(r8), DIMENSION(:,:), ALLOCATABLE :: mx0
       COMPLEX(r8), DIMENSION(:), ALLOCATABLE :: vx0
 
@@ -346,6 +347,24 @@ SUBROUTINE dcon_run
       IF(MRE_flag)THEN
          CALL read_var_len(Zeff,Zeff_)
          CALL read_var_len(psi_N_Zeff,psi_N_Zeff_)
+c        one Zeff value (default 1.52) without psi_N_Zeff: flat profile.
+         IF(.NOT.ALLOCATED(psi_N_Zeff_))THEN
+            zeff0=1.52_r8
+            IF(ALLOCATED(Zeff_))THEN
+               IF(SIZE(Zeff_) > 1)
+     $              CALL program_stop("A Zeff profile needs psi_N_Zeff")
+               zeff0=Zeff_(1)
+               DEALLOCATE(Zeff_)
+            ENDIF
+            ALLOCATE(Zeff_(4),psi_N_Zeff_(4))
+            Zeff_=zeff0
+            psi_N_Zeff_=(/0.0_r8,1.0_r8/3,2.0_r8/3,1.0_r8/)
+         ENDIF
+         IF(.NOT.ALLOCATED(Zeff_) .OR. SIZE(Zeff_) /= SIZE(psi_N_Zeff_)
+     $     .OR. SIZE(Zeff_) < 4)
+     $     CALL program_stop("Zeff, psi_N_Zeff: equal sizes, >= 4")
+         IF(ANY(psi_N_Zeff_(2:) <= psi_N_Zeff_(:SIZE(psi_N_Zeff_)-1)))
+     $     CALL program_stop("psi_N_Zeff must be strictly increasing")
          CALL spline_alloc(mreterms,mpsi,30)
          mreterms%xs=sq%xs
          mreterms%fs=0
diff --git a/rdcon/mercier.f b/rdcon/mercier.f
index b3e03ed2..277894db 100644
--- a/rdcon/mercier.f
+++ b/rdcon/mercier.f
@@ -43,14 +43,10 @@ SUBROUTINE mercier_scan
 c-----------------------------------------------------------------------
       IF(MRE_flag)THEN
          CALL spline_alloc(ff,mtheta,22)
-         IF(ALLOCATED(Zeff_) .AND. ALLOCATED(psi_N_Zeff_))THEN
-            CALL spline_alloc(Zeff_spline,SIZE(psi_N_Zeff_)-1,1)
-            Zeff_spline%xs=psi_N_Zeff_
-            Zeff_spline%fs(:,1)=Zeff_
-            CALL spline_fit(Zeff_spline,"extrap")
-         ELSE
-            CALL program_stop("Provide Zeff profile values")
-         ENDIF
+         CALL spline_alloc(Zeff_spline,SIZE(psi_N_Zeff_)-1,1)
+         Zeff_spline%xs=psi_N_Zeff_
+         Zeff_spline%fs(:,1)=Zeff_
+         CALL spline_fit(Zeff_spline,"extrap")
       ELSE
          CALL spline_alloc(ff,mtheta,5)
       ENDIF