diff --git a/msg_helpers.go b/msg_helpers.go
index 7a4184dc..ecd9280f 100644
--- a/msg_helpers.go
+++ b/msg_helpers.go
@@ -25,12 +25,13 @@ func unpackDataA(msg []byte, off int) (net.IP, int, error) {
 }
 
 func packDataA(a net.IP, msg []byte, off int) (int, error) {
-	// It must be a slice of 4, even if it is 16, we encode only the first 4
-	if off+net.IPv4len > len(msg) {
-		return len(msg), &Error{err: "overflow packing a"}
-	}
 	switch len(a) {
 	case net.IPv4len, net.IPv6len:
+		// It must be a slice of 4, even if it is 16, we encode only the first 4
+		if off+net.IPv4len > len(msg) {
+			return len(msg), &Error{err: "overflow packing a"}
+		}
+
 		copy(msg[off:], a.To4())
 		off += net.IPv4len
 	case 0:
@@ -51,12 +52,12 @@ func unpackDataAAAA(msg []byte, off int) (net.IP, int, error) {
 }
 
 func packDataAAAA(aaaa net.IP, msg []byte, off int) (int, error) {
-	if off+net.IPv6len > len(msg) {
-		return len(msg), &Error{err: "overflow packing aaaa"}
-	}
-
 	switch len(aaaa) {
 	case net.IPv6len:
+		if off+net.IPv6len > len(msg) {
+			return len(msg), &Error{err: "overflow packing aaaa"}
+		}
+
 		copy(msg[off:], aaaa)
 		off += net.IPv6len
 	case 0:
diff --git a/msg_test.go b/msg_test.go
index 616e44db..55cc87a1 100644
--- a/msg_test.go
+++ b/msg_test.go
@@ -306,3 +306,20 @@ func TestPackUnpackManyCompressionPointers(t *testing.T) {
 		}
 	}
 }
+
+func TestLenDynamicA(t *testing.T) {
+	for _, rr := range []RR{
+		testRR("example.org. A"),
+		testRR("example.org. AAAA"),
+		testRR("example.org. L32"),
+	} {
+		msg := make([]byte, Len(rr))
+		off, err := PackRR(rr, msg, 0, nil, false)
+		if err != nil {
+			t.Fatalf("PackRR failed for %T: %v", rr, err)
+		}
+		if off != len(msg) {
+			t.Errorf("Len(rr) wrong for %T: Len(rr) = %d, PackRR(rr) = %d", rr, len(msg), off)
+		}
+	}
+}
diff --git a/types_generate.go b/types_generate.go
index aa05a085..cbb4a00c 100644
--- a/types_generate.go
+++ b/types_generate.go
@@ -196,9 +196,9 @@ func main() {
 			case st.Tag(i) == `dns:"any"`:
 				o("l += len(rr.%s)\n")
 			case st.Tag(i) == `dns:"a"`:
-				o("l += net.IPv4len // %s\n")
+				o("if len(rr.%s) != 0 { l += net.IPv4len }\n")
 			case st.Tag(i) == `dns:"aaaa"`:
-				o("l += net.IPv6len // %s\n")
+				o("if len(rr.%s) != 0 { l += net.IPv6len }\n")
 			case st.Tag(i) == `dns:"txt"`:
 				o("for _, t := range rr.%s { l += len(t) + 1 }\n")
 			case st.Tag(i) == `dns:"uint48"`:
diff --git a/ztypes.go b/ztypes.go
index 19a542d3..495a83e3 100644
--- a/ztypes.go
+++ b/ztypes.go
@@ -240,12 +240,16 @@ func (rr *X25) Header() *RR_Header        { return &rr.Hdr }
 // len() functions
 func (rr *A) len(off int, compression map[string]struct{}) int {
 	l := rr.Hdr.len(off, compression)
-	l += net.IPv4len // A
+	if len(rr.A) != 0 {
+		l += net.IPv4len
+	}
 	return l
 }
 func (rr *AAAA) len(off int, compression map[string]struct{}) int {
 	l := rr.Hdr.len(off, compression)
-	l += net.IPv6len // AAAA
+	if len(rr.AAAA) != 0 {
+		l += net.IPv6len
+	}
 	return l
 }
 func (rr *AFSDB) len(off int, compression map[string]struct{}) int {
@@ -364,8 +368,10 @@ func (rr *KX) len(off int, compression map[string]struct{}) int {
 }
 func (rr *L32) len(off int, compression map[string]struct{}) int {
 	l := rr.Hdr.len(off, compression)
-	l += 2           // Preference
-	l += net.IPv4len // Locator32
+	l += 2 // Preference
+	if len(rr.Locator32) != 0 {
+		l += net.IPv4len
+	}
 	return l
 }
 func (rr *L64) len(off int, compression map[string]struct{}) int {