diff --git a/lib/flat4.f90 b/lib/flat4.f90
index f9d4c72f2..fd96a3aba 100644
--- a/lib/flat4.f90
+++ b/lib/flat4.f90
@@ -28,7 +28,7 @@ subroutine flat4(s,npts,nflatten)
      call pctile(s(ia),ib-ia+1,npct,base) !Find lowest npct of points in segment
      do i=ia,ib
         if(s(i).le.base) then
-           k=k+1                      !Save these "lower envelope" points
+           if (k.lt.1000) k=k+1       !Save these "lower envelope" points
            x(k)=i-i0
            y(k)=s(i)
         endif