diff --git a/ac_test.go b/ac_test.go
index 57f2b8e..2c7dfe2 100644
--- a/ac_test.go
+++ b/ac_test.go
@@ -204,6 +204,14 @@ func TestACBlices(t *testing.T) {
 	}
 }
 
+func TestNonASCIIDictionary(t *testing.T) {
+	dict := []string{"hello world", "こんにちは世界"}
+	_, err := CompileString(dict)
+	if err != nil {
+		t.Errorf("error compiling matcher: %s", err)
+	}
+}
+
 var (
 	source1  = "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/30.0.1599.101 Safari/537.36"
 	source1b = []byte(source1)
diff --git a/acascii/ac.go b/acascii/ac.go
index b3e1979..006db35 100644
--- a/acascii/ac.go
+++ b/acascii/ac.go
@@ -11,10 +11,14 @@ package acascii
 
 import (
 	"container/list"
+	"errors"
 )
 
 const maxchar = 128
 
+// ErrNotASCII is returned when the dictionary input is not ASCII
+var ErrNotASCII = errors.New("non-ASCII input")
+
 // A node in the trie structure used to implement Aho-Corasick
 type node struct {
 	root bool // true if this is the root
@@ -88,7 +92,7 @@ func (m *Matcher) getFreeNode() *node {
 
 // buildTrie builds the fundamental trie structure from a set of
 // blices.
-func (m *Matcher) buildTrie(dictionary [][]byte) {
+func (m *Matcher) buildTrie(dictionary [][]byte) error {
 
 	// Work out the maximum size for the trie (all dictionary entries
 	// are distinct plus the root). This is used to preallocate memory
@@ -112,6 +116,9 @@ func (m *Matcher) buildTrie(dictionary [][]byte) {
 		n := m.root
 		for i, b := range blice {
 			idx := int(b)
+			if idx >= maxchar {
+				return ErrNotASCII 
+			}
 			c := n.child[idx]
 
 			if c == nil {
@@ -185,10 +192,11 @@ func (m *Matcher) buildTrie(dictionary [][]byte) {
 	}
 
 	m.trie = m.trie[:m.extent]
+	return nil
 }
 
 // buildTrieString builds the fundamental trie structure from a []string
-func (m *Matcher) buildTrieString(dictionary []string) {
+func (m *Matcher) buildTrieString(dictionary []string) error {
 
 	// Work out the maximum size for the trie (all dictionary entries
 	// are distinct plus the root). This is used to preallocate memory
@@ -212,6 +220,10 @@ func (m *Matcher) buildTrieString(dictionary []string) {
 	for _, blice := range dictionary {
 		n := m.root
 		for i := 0; i < len(blice); i++ {
+			index := int(blice[i])
+			if index >= maxchar {
+				return ErrNotASCII
+			}
 			b := int(blice[i])
 			c := n.child[b]
 			if c == nil {
@@ -285,13 +297,15 @@ func (m *Matcher) buildTrieString(dictionary []string) {
 	}
 
 	m.trie = m.trie[:m.extent]
+	return nil
 }
 
 // Compile creates a new Matcher using a list of []byte
 func Compile(dictionary [][]byte) (*Matcher, error) {
 	m := new(Matcher)
-	m.buildTrie(dictionary)
-	// no error for now
+	if err := m.buildTrie(dictionary); err != nil {
+		return nil, err
+	}
 	return m, nil
 }
 
@@ -308,7 +322,9 @@ func MustCompile(dictionary [][]byte) *Matcher {
 // of strings (this is a helper to make initialization easy)
 func CompileString(dictionary []string) (*Matcher, error) {
 	m := new(Matcher)
-	m.buildTrieString(dictionary)
+	if err := m.buildTrieString(dictionary); err != nil {
+		return nil, err
+	}	
 	return m, nil
 }
 
@@ -415,7 +431,7 @@ func (m *Matcher) Match(in []byte) bool {
 	for _, b := range in {
 		c := int(b)
 		if c > maxchar {
-			c = 0
+		   c = 0	
 		}
 		if !n.root && n.child[c] == nil {
 			n = n.fails[c]
@@ -443,7 +459,7 @@ func (m *Matcher) MatchString(in string) bool {
 	for idx := 0; idx < slen; idx++ {
 		c := int(in[idx])
 		if c >= maxchar {
-			c = 0
+			c = 0	
 		}
 		if !n.root && n.child[c] == nil {
 			n = n.fails[c]
diff --git a/acascii/ac_test.go b/acascii/ac_test.go
index 8bebf44..904f5c3 100644
--- a/acascii/ac_test.go
+++ b/acascii/ac_test.go
@@ -204,6 +204,14 @@ func TestACBlices(t *testing.T) {
 	}
 }
 
+func TestNonASCIIDictionary(t *testing.T) {
+	dict := []string{"hello world", "こんにちは世界"}
+	_, err := CompileString(dict)
+	if err == nil {
+		t.Errorf("expected error compiling ASCII matcher")
+	}
+}
+
 var (
 	source1  = "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/30.0.1599.101 Safari/537.36"
 	source1b = []byte(source1)