diff --git a/lib/spec64.f90 b/lib/spec64.f90
index 73be44d18..048bf6700 100644
--- a/lib/spec64.f90
+++ b/lib/spec64.f90
@@ -20,6 +20,8 @@ subroutine spec64(c0,npts,nsps,mode_q65,jpk,s3,LL,NN)
      j=j+1
      ja=(k-1)*nsps + jpk
      jb=ja+nsps-1
+     if(ja.lt.0) ja=0
+     if(jb.gt.npts-1) jb=npts-1
      cs(0:nfft-1)=c0(ja:jb)
      call four2a(cs,nsps,1,-1,1)             !c2c FFT to frequency
      do ii=1,LL