diff --git a/doc/bird.sgml b/doc/bird.sgml
index e85ebf18..761c5396 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -1196,27 +1196,29 @@ foot).
 	This type can hold a network prefix consisting of IP address, prefix
 	length and several other values. This is the key in route tables.
 
-	Prefices may be of several types, which can be determined by the special
-	operator <cf/.type/ of type <m/enum/. The type may be:
+	Prefixes may be of several types, which can be determined by the special
+	operator <cf/.type/. The type may be:
 
-	<cf/NET_IP4/ and <cf/NET_IP6/ prefices hold an IP prefix. The literals are 
-	written as <cf><m/ipaddress//<m/pxlen/</cf>,
+	<cf/NET_IP4/ and <cf/NET_IP6/ prefixes hold an IP prefix. The literals
+	are written as <cf><m/ipaddress//<m/pxlen/</cf>,
 	or <cf><m>ipaddress</m>/<m>netmask</m></cf>. There are two special
-	operators on IP prefices: <cf/.ip/ which extracts the IP address from the
-	pair, and <cf/.len/, which separates prefix length from the pair.
+	operators on IP prefixes: <cf/.ip/ which extracts the IP address from
+	the pair, and <cf/.len/, which separates prefix length from the pair.
 	So <cf>1.2.0.0/16.len = 16</cf> is true.
 
-	<cf/NET_VPN4/ and <cf/NET_VPN6/ prefices hold an IP prefix with
-	VPN Route Distinguisher (<rfc id="4364">). They support the same special
-	operators as IP prefices, and also <cf/.rd/ which extracts the Route Distinguisher.
-	Their literals are written as <cf><m/vpnrd/ <m/ipprefix/</cf>
+	<cf/NET_VPN4/ and <cf/NET_VPN6/ prefixes hold an IP prefix with VPN
+	Route Distinguisher (<rfc id="4364">). They support the same special
+	operators as IP prefixes, and also <cf/.rd/ which extracts the Route
+	Distinguisher. Their literals are written
+	as <cf><m/vpnrd/ <m/ipprefix/</cf>
 
-	<cf/NET_ROA4/ and <cf/NET_ROA6/ prefices hold an IP prefix range together
-	with an ASN. They support the same special operators as IP prefices, and also
-	<cf/.maxlen/ which extracts maximal prefix length, and <cf/.asn/ which extracts the ASN.
+	<cf/NET_ROA4/ and <cf/NET_ROA6/ prefixes hold an IP prefix range
+	together with an ASN. They support the same special operators as IP
+	prefixes, and also <cf/.maxlen/ which extracts maximal prefix length,
+	and <cf/.asn/ which extracts the ASN.
 
-	<cf/NET_FLOW4/ and <cf/NET_FLOW6/ hold an IP prefix together with
-	a flowspec rule. Filters currently don't support flowspec parsing.
+	<cf/NET_FLOW4/ and <cf/NET_FLOW6/ hold an IP prefix together with a
+	flowspec rule. Filters currently don't support flowspec parsing.
 
 	<tag><label id="type-ec">ec</tag>
 	This is a specialized type used to represent BGP extended community
diff --git a/filter/filter.c b/filter/filter.c
index a3caaf96..616f9e2a 100644
--- a/filter/filter.c
+++ b/filter/filter.c
@@ -1227,11 +1227,10 @@ interpret(struct f_inst *what)
     ONEARG;
     if (v1.type != T_NET)
       runtime( "Prefix expected" );
-    res.type = T_RD;
-    if ((1 << v1.val.net->type) & (NB_VPN4 | NB_VPN6))
-      res.val.ec = net_rd(v1.val.net);
-    else
+    if (!net_is_vpn(v1.val.net))
       runtime( "VPN address expected" );
+    res.type = T_RD;
+    res.val.ec = net_rd(v1.val.net);
     break;
   case P('a','f'):	/* Get first ASN from AS PATH */
     ONEARG;
diff --git a/lib/net.c b/lib/net.c
index 2bf4049e..c29ed299 100644
--- a/lib/net.c
+++ b/lib/net.c
@@ -81,14 +81,14 @@ net_format(const net_addr *N, char *buf, int buflen)
   case NET_VPN4:
     {
     int c = rd_format(n->vpn4.rd, buf, buflen);
-    buf += c; buflen -= c;
+    ADVANCE(buf, buflen, c);
     return bsnprintf(buf, buflen, " %I4/%d", n->vpn4.prefix, n->vpn4.pxlen);
     }
   case NET_VPN6:
     {
     /* XXX: RD format is specified for VPN4; not found any for VPN6, reusing the same as for VPN4 */
     int c = rd_format(n->vpn6.rd, buf, buflen);
-    buf += c; buflen -= c;
+    ADVANCE(buf, buflen, c);
     return bsnprintf(buf, buflen, " %I6/%d", n->vpn6.prefix, n->vpn6.pxlen);
     }
   case NET_ROA4: