diff --git a/src/csf/sigma_vector.irp.f b/src/csf/sigma_vector.irp.f
index 4d409f50..beb827ba 100644
--- a/src/csf/sigma_vector.irp.f
+++ b/src/csf/sigma_vector.irp.f
@@ -52,10 +52,15 @@
           ncfgpersomo = cfg_seniority_index(i+2)
       else
         k = 0
-        do while(cfg_seniority_index(i+2+k) < ncfgpersomo)
-          k = k + 2
-          ncfgpersomo = cfg_seniority_index(i+2+k)
-        enddo
+        if ((i+2+k) < size(cfg_seniority_index,1)) then
+          do while(cfg_seniority_index(i+2+k) < ncfgpersomo)
+            k = k + 2
+            if ((i+2+k) >= size(cfg_seniority_index,1)) then
+              exit
+            endif
+            ncfgpersomo = cfg_seniority_index(i+2+k)
+          enddo
+        endif
       endif
     endif
     ncfg = ncfgpersomo - ncfgprev
@@ -75,10 +80,15 @@
       ncfgprev = cfg_seniority_index(i+2)
     else
       k = 0
-      do while(cfg_seniority_index(i+2+k) < ncfgprev)
-        k = k + 2
-        ncfgprev = cfg_seniority_index(i+2+k)
-      enddo
+      if ((i+2+k) < size(cfg_seniority_index,1)) then
+        do while(cfg_seniority_index(i+2+k) < ncfgprev)
+          k = k + 2
+          if ((i+2+k) >= size(cfg_seniority_index,1)) then
+            exit
+          endif
+          ncfgprev = cfg_seniority_index(i+2+k)
+        enddo
+      endif
     endif
   enddo
 END_PROVIDER