diff --git a/aezeed/cipherseed.go b/aezeed/cipherseed.go
index 112a5a99..50ad8f2d 100644
--- a/aezeed/cipherseed.go
+++ b/aezeed/cipherseed.go
@@ -6,7 +6,6 @@ import (
 	"encoding/binary"
 	"hash/crc32"
 	"io"
-	"strings"
 	"time"
 
 	"github.com/Yawning/aez"
@@ -506,8 +505,13 @@ func (m *Mnemonic) Decipher(pass []byte) ([DecipheredCipherSeedSize]byte, error)
 	// Before we attempt to map the mnemonic back to the original
 	// ciphertext, we'll ensure that all the word are actually a part of
 	// the current default word list.
+	wordDict := make(map[string]struct{}, len(defaultWordList))
+	for _, word := range defaultWordList {
+		wordDict[word] = struct{}{}
+	}
+
 	for i, word := range m {
-		if !strings.Contains(englishWordList, word) {
+		if _, ok := wordDict[word]; !ok {
 			emptySeed := [DecipheredCipherSeedSize]byte{}
 			return emptySeed, ErrUnknownMnenomicWord{
 				Word:  word,
diff --git a/aezeed/cipherseed_test.go b/aezeed/cipherseed_test.go
index 1d74e88c..cd8e4366 100644
--- a/aezeed/cipherseed_test.go
+++ b/aezeed/cipherseed_test.go
@@ -550,6 +550,22 @@ func TestDecipherUnknownMnenomicWord(t *testing.T) {
 		t.Fatalf("wrong index detected: expected %v, got %v",
 			randIndex, wordErr.Index)
 	}
+
+	// If the mnemonic includes a word that is not in the englishList
+	// it fails, even when it is a substring of a valid word
+	// Example: `heart` is in the list, `hear` is not
+	mnemonic[randIndex] = "hear"
+
+	// If we attempt to map back to the original cipher seed now, then we
+	// should get ErrUnknownMnenomicWord.
+	_, err = mnemonic.ToCipherSeed(pass)
+	if err == nil {
+		t.Fatalf("expected ErrUnknownMnenomicWord error")
+	}
+	_, ok = err.(ErrUnknownMnenomicWord)
+	if !ok {
+		t.Fatalf("expected ErrUnknownMnenomicWord instead got %T", err)
+	}
 }
 
 // TestDecipherIncorrectMnemonic tests that if we obtain a cipherseed, but then
@@ -582,7 +598,6 @@ func TestDecipherIncorrectMnemonic(t *testing.T) {
 	if err != ErrIncorrectMnemonic {
 		t.Fatalf("expected ErrIncorrectMnemonic error")
 	}
-
 }
 
 // TODO(roasbeef): add test failure checksum fail is modified, new error