diff --git a/lib/77bit/packjt77.f90 b/lib/77bit/packjt77.f90
index 1f9ed9a19..90e17add2 100644
--- a/lib/77bit/packjt77.f90
+++ b/lib/77bit/packjt77.f90
@@ -519,6 +519,8 @@ subroutine unpack77(c77,nrx,msg,unpk77_success)
           ' R'//crpt//' '//cfield
      if(itu.eq.1 .and. ir.eq.1) msg='TU; '//trim(call_1)//' '//trim(call_2)//     &
           ' R'//crpt//' '//cfield
+  else if(i3.ge.6) then ! i3 values 6 and 7 are not yet defined
+     unpk77_success=.false.
   endif
 !  if(msg(1:4).eq.'CQ <') unpk77_success=.false.
 
diff --git a/lib/ft4_decode.f90 b/lib/ft4_decode.f90
index 6b561c76b..3e8ed3134 100644
--- a/lib/ft4_decode.f90
+++ b/lib/ft4_decode.f90
@@ -429,7 +429,8 @@ contains
                      message77=mod(message77+rvec,2) ! remove rvec scrambling
                      write(c77,'(77i1)') message77(1:77)
                      call unpack77(c77,1,message,unpk77_success)
-                     if(unpk77_success.and.dosubtract) then
+                     if(.not.unpk77_success) exit
+                     if(dosubtract) then
                         call get_ft4_tones_from_77bits(message77,i4tone)
                         dt=real(ibest)/666.67
                         call timer('subtract',0)