diff --git a/decode.go b/decode.go
index bd18166..08401fb 100644
--- a/decode.go
+++ b/decode.go
@@ -174,10 +174,7 @@ func (d *decodeState) readSize() int64 {
 	return x
 }
 
-var i int
-
 func (d *decodeState) readValue(size int64, v reflect.Value) {
-	i++
 	if !v.IsValid() {
 		_, err := d.r.Seek(size, 1)
 		if err != nil {
@@ -187,7 +184,15 @@ func (d *decodeState) readValue(size int64, v reflect.Value) {
 	}
 
 	switch v.Kind() {
-	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		b := make([]byte, size)
+		_, err := d.r.Read(b)
+		if err != nil {
+			d.error(err)
+		}
+		v.SetInt(unmarshalInt(b))
+
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
 		b := make([]byte, size)
 		_, err := d.r.Read(b)
 		if err != nil {
@@ -358,6 +363,15 @@ func indirect(v reflect.Value) reflect.Value {
 	return v
 }
 
+func unmarshalInt(b []byte) int64 {
+	x := int64(b[0])
+	for _, c := range b[1:] {
+		x <<= 8
+		x += int64(c)
+	}
+	return x
+}
+
 func unmarshalUint(b []byte) uint64 {
 	x := uint64(b[0])
 	for _, c := range b[1:] {
diff --git a/decoder.go b/decoder.go
index a21b389..79eb37c 100644
--- a/decoder.go
+++ b/decoder.go
@@ -8,7 +8,6 @@ import (
 	"io"
 )
 
-
 type Decoder struct {
 	r   io.ReadSeeker
 	err error
diff --git a/ebml_test.go b/ebml_test.go
index bc76659..0e33a48 100644
--- a/ebml_test.go
+++ b/ebml_test.go
@@ -1,6 +1,7 @@
 package ebml
 
 import (
+	"bytes"
 	"reflect"
 	"testing"
 )
@@ -15,18 +16,24 @@ func TestMatroskaEBMLHeader(t *testing.T) {
 	headerA.DocTypeVersion = 1
 	headerA.DocTypeReadVersion = 1
 
-	b, err := Marshal(headerA)
+	dst := new(bytes.Buffer)
+	enc := NewEncoder(dst)
+
+	err := enc.Encode(headerA)
 	if err != nil {
 		t.Fatal("Marshal:", err)
 	}
 
+	src := bytes.NewReader(dst.Bytes())
+	dec := NewDecoder(src)
+
 	var headerB Header
-	err = Unmarshal(b, &headerB)
+	err = dec.Decode(&headerB)
 	if err != nil {
 		t.Fatal("Unmarshal:", err)
 	}
 
 	if !reflect.DeepEqual(headerA, headerB) {
-		t.Fatalf("Marshal -> Unmarshal: marshaled %v to %x, but unmarshaled %v", headerA, b, headerB)
+		t.Fatalf("Marshal -> Unmarshal: marshaled %v to %x, but unmarshaled %v", headerA, dst.Bytes(), headerB)
 	}
 }
diff --git a/encode.go b/encode.go
index afe097a..31c75d7 100644
--- a/encode.go
+++ b/encode.go
@@ -215,7 +215,9 @@ func fieldByIndex(v reflect.Value, index []int) reflect.Value {
 	return v
 }
 
-func parseIdTag(s string) ([]byte, error) {
+// ParseId marshals a hexadecimal number into a byte slice.
+// It can be used to check that an Id has the proper width bit set.
+func ParseId(s string) ([]byte, error) {
 	x, err := strconv.ParseUint(s, 16, 32)
 	if err != nil {
 		return nil, err
@@ -428,7 +430,7 @@ func typeFields(t reflect.Type) []field {
 				if tag == "" {
 					continue
 				}
-				id, err := parseIdTag(tag)
+				id, err := ParseId(tag)
 				if err != nil {
 					panic(err.Error())
 				}
diff --git a/idtest/idtest.go b/idtest/idtest.go
new file mode 100644
index 0000000..03e076d
--- /dev/null
+++ b/idtest/idtest.go
@@ -0,0 +1,42 @@
+// idtest is a utility for checking the validity of EBML ids
+package main
+
+import (
+	"fmt"
+	"git.gitorious.org/go-ebml/ebml.git"
+	"os"
+)
+
+func main() {
+	for _, s := range os.Args[1:] {
+		b, err := ebml.ParseId(s)
+		if err != nil {
+			fmt.Println(err)
+			continue
+		}
+
+		c := b[0]
+		var good bool
+		switch l := len(b); l {
+		case 1:
+			good = c <= 0xff && c > 0x80
+		case 2:
+			good = c <= 0x7f && c > 0x40
+		case 3:
+			good = c <= 0x3f && c > 0x20
+		case 4:
+			good = c <= 0x1f && c > 0x10
+		}
+
+		if good {
+			fmt.Print("Id is good: ")
+		} else {
+			fmt.Print("Id is bad:  ")
+		}
+		fmt.Printf("% 11x ", b)
+		for _, c := range b {
+			fmt.Printf(".%08b", c)
+		}
+		fmt.Println()
+	}
+}