diff --git a/lib/ft8/sync8.f90 b/lib/ft8/sync8.f90
index 6191f5265..f7510758f 100644
--- a/lib/ft8/sync8.f90
+++ b/lib/ft8/sync8.f90
@@ -105,6 +105,8 @@ subroutine sync8(dd,nfa,nfb,syncmin,nfqso,ldecode77,maxcand,s,candidate,ncand,sb
     iz=ib-ia+1
     call indexx(red(ia:ib),iz,indx)
     ibase=indx(nint(0.40*iz)) - 1 + ia
+    if(ibase.lt.1) ibase=1
+    if(ibase.gt.nh1) ibase=nh1
     base=red(ibase)
     red=red/base