diff --git a/dns.packer.js b/dns.packer.js
index 8a5e283..60824b7 100644
--- a/dns.packer.js
+++ b/dns.packer.js
@@ -16,6 +16,10 @@ var dnspack = exports.DNS_PACKER = {
     var header = dnspack.packHeader(packet.header); // 2 bytes
     total = 12;                                     // 2+2+2+2 bytes ({qd,an,ns,ar}count)
 
+    if (!id) {
+      throw new Error("'id' should be set to a crypto random id");
+    }
+
     dv.setUint16(0, id, false);
     dv.setUint16(2, header, false);
     dv.setUint16(4, packet.question.length, false);
@@ -135,17 +139,25 @@ var dnspack = exports.DNS_PACKER = {
 
 , packHeader: function(h) {
     var val = 0;
+    var req = h.opcode;
+    var res = h.qr || h.aa || h.ra || h.rcode;
 
-    val += (h.qr     << 15) & 0x8000;
-    val += (h.opcode << 11) & 0x7800;
-    val += (h.aa     << 10) &  0x400;
-    val += (h.tc     <<  9) &  0x200;
-    val += (h.rd     <<  8) &  0x100;
-    val += (h.ra     <<  7) &   0x80;
-    val += (h.res1   <<  6) &   0x40;
-    val += (h.res2   <<  5) &   0x20;
-    val += (h.res3   <<  4) &   0x10;
-    val += h.rcode          &    0xF;
+    if (req && res) {
+      throw new Error("Both request-only and response-only headers are set: " + JSON.stringify(h));
+    }
+
+    // TODO check if is request or response
+
+    val += ((h.qr || 0)     << 15) & 0x8000;
+    val += ((h.opcode || 0) << 11) & 0x7800;
+    val += ((h.aa || 0)     << 10) &  0x400;
+    val += ((h.tc || 0)     <<  9) &  0x200;
+    val += ((h.rd || 0)     <<  8) &  0x100;
+    val += ((h.ra || 0)     <<  7) &   0x80;
+    val += ((h.res1 || 0)   <<  6) &   0x40;
+    val += ((h.res2 || 0)   <<  5) &   0x20;
+    val += ((h.res3 || 0)   <<  4) &   0x10;
+    val += (h.rcode || 0)          &    0xF;
 
     return val;
   }