diff --git a/uniuri.go b/uniuri.go
index 0e9ee1b..41e5719 100644
--- a/uniuri.go
+++ b/uniuri.go
@@ -48,14 +48,18 @@ func NewLen(length int) string {
 func NewLenChars(length int, chars []byte) string {
 	b := make([]byte, length)
 	r := make([]byte, length+(length/4)) // storage for random bytes.
-	clen := byte(len(chars))
-	maxrb := byte(256 - (256 % len(chars)))
+	clen := len(chars)
+	if clen > 256 {
+		panic("uniuri: maximum length of charset for NewLenChars is 256")
+	}
+	maxrb := 256 - (256 % len(chars))
 	i := 0
 	for {
 		if _, err := io.ReadFull(rand.Reader, r); err != nil {
 			panic("error reading from random source: " + err.Error())
 		}
-		for _, c := range r {
+		for _, byte := range r {
+			c := int(byte)
 			if c > maxrb {
 				// Skip this number to avoid modulo bias.
 				continue
diff --git a/uniuri_test.go b/uniuri_test.go
index ff9a83d..17b3ba1 100644
--- a/uniuri_test.go
+++ b/uniuri_test.go
@@ -50,4 +50,20 @@ func TestNewLenChars(t *testing.T) {
 	}
 	// Check that only allowed characters are present
 	validateChars(t, u, chars)
+
+	// Check that two generated strings are different
+	u2 := NewLenChars(length, chars)
+	if u == u2 {
+		t.Fatalf("not unique: %q and %q", u, u2)
+	}
+}
+
+func TestNewLenCharsMaxLength(t *testing.T) {
+	defer func() {
+		if r := recover(); r == nil {
+			t.Fatal("didn't panic")
+		}
+	}()
+	chars := make([]byte, 257)
+	NewLenChars(32, chars)
 }