diff --git a/commands/CmdE.c b/commands/CmdE.c
index 6f9f7912..0054fdf7 100644
--- a/commands/CmdE.c
+++ b/commands/CmdE.c
@@ -1085,9 +1085,9 @@ CmdExtract(w, cmd)
 
 	    /* argv[2] is a halo distance */
 	    dist = cmdParseCoord(w, argv[2], TRUE, TRUE);
-	    if (dist <= 0)
+	    if (dist < 0)
 	    {
-		TxError("Bad halo distance.  Halo must be strictly positive.");
+		TxError("Bad halo distance.  Halo must be non-negative.");
 		return;
 	    }
 	    else
diff --git a/extract/ExtCouple.c b/extract/ExtCouple.c
index c4c433c1..f883d88e 100644
--- a/extract/ExtCouple.c
+++ b/extract/ExtCouple.c
@@ -1180,7 +1180,7 @@ extSideOverlapHalo(tp, esws)
 	    }
 	    if (rtp != rbp)
 	    {
-		efflength = (cfrac * (double)length) * (1.0 - sov.so_coupfrac);
+		efflength = (cfrac - sov.so_coupfrac) * (double)length;
 		cap += e->ec_cap * efflength;
 
 		subfrac += sov.so_subfrac;	/* Just add the shielded fraction */
@@ -1209,7 +1209,7 @@ extSideOverlapHalo(tp, esws)
 	    if (DBIsContact(outtype))
 		outtype = DBPlaneToResidue(outtype, esws->plane_of_boundary);
 
-	    efflength = (sfrac * (double)length) * (1.0 - subfrac);
+	    efflength = (sfrac - subfrac) * (double)length;
 	    subcap = ExtCurStyle->exts_perimCap[ta][0] * efflength;
 	    rbp->nreg_cap -= subcap;
 	    /* Ignore residual error at ~zero zeptoFarads.  Probably	*/