diff --git a/pmap.go b/pmap.go
index fe09ee0..1c5ed8a 100644
--- a/pmap.go
+++ b/pmap.go
@@ -5,9 +5,11 @@ import (
 	"math/bits"
 )
 
-const deg = 16 // branch factor of nodes
-const log2deg = 4
-const mask = 0b1111
+const (
+	nodeDegree = 16 // branch factor of nodes
+	nodeShift  = 4
+	nodeMask   = 0b1111
+)
 
 type Key = interface{}
 type Value = interface{}
@@ -27,7 +29,7 @@ type pmap struct {
 
 // A Map implemented as a hashed trie
 type node struct {
-	child  [deg]interface{}
+	child  [nodeDegree]interface{}
 	bitmap uint32
 }
 
@@ -50,8 +52,9 @@ func (p pmap) Len() int {
 }
 
 func (p pmap) Get(k Key) (Value, bool) {
+	var zero Key
 	h := hash(k) // TODO: p.hash
-	return lookup(p.root, 0, h, k)
+	return lookup(p.root, 0, h, k, zero)
 }
 
 func (p pmap) Set(k Key, v Value) Map {
@@ -73,8 +76,8 @@ func (p pmap) Set(k Key, v Value) Map {
 	if n == nil {
 		n = &node{}
 	}
-	n.child[h&mask] = leaf{k, v}
-	n.bitmap = 1 << (h & mask)
+	n.child[h&nodeMask] = leaf{k, v}
+	n.bitmap = 1 << (h & nodeMask)
 	p.root = n
 	p.len++
 	return p
@@ -92,7 +95,7 @@ func hash(k Key) uint32 {
 }
 
 func (m *node) getNode(shift, hash uint32, key Key) interface{} {
-	i := hash >> shift & mask
+	i := hash >> shift & nodeMask
 	return m.child[i]
 }
 
@@ -108,21 +111,21 @@ func (m *collision) getNode(hash uint32, key Key) interface{} {
 	return nil
 }
 
-func lookup(root interface{}, shift, hash uint32, key Key) (Value, bool) {
+func lookup(root interface{}, shift, hash uint32, key Key, zero Key) (Value, bool) {
 	cur := root
 	for {
 		switch n := cur.(type) {
 		case nil:
-			return nil, false
+			return zero, false
 		case leaf:
 			if n.k == key {
 				return n.v, true
 			} else {
-				return nil, false
+				return zero, false
 			}
 		case *node:
 			cur = n.getNode(shift, hash, key)
-			shift += log2deg
+			shift += nodeShift
 		case *collision:
 			cur = n.getNode(hash, key)
 		default:
@@ -137,7 +140,7 @@ func singleton(key Key, val Value, hash, shift uint32) *node {
 
 func newnode(child interface{}, hash, shift uint32) *node {
 	n := &node{}
-	idx := hash >> shift & mask
+	idx := hash >> shift & nodeMask
 	n.child[idx] = child
 	n.bitmap = 1 << idx
 	return n
@@ -181,9 +184,9 @@ func insert(n interface{}, shift, hash uint32, key Key, val Value, hashFn hashFu
 				c = leaf{key, val}
 				added = true
 			} else {
-				c = _insert(c, shift+log2deg)
+				c = _insert(c, shift+nodeShift)
 			}
-			idx := hash >> shift & mask
+			idx := hash >> shift & nodeMask
 			x := &node{child: n.child, bitmap: n.bitmap}
 			x.child[idx] = c
 			x.bitmap |= 1 << idx