diff --git a/lib/key-encoder.js b/lib/key-encoder.js
index 286e522..ef2ea53 100644
--- a/lib/key-encoder.js
+++ b/lib/key-encoder.js
@@ -85,7 +85,7 @@ KeyEncoder.prototype.encodePrivate = function(privateKey, originalFormat, destin
 
     /* Parse the incoming private key and convert it to a private key object */
     if (originalFormat === 'raw') {
-        if (!typeof privateKey === 'string') {
+        if (typeof privateKey !== 'string') {
             throw 'private key must be a string'
         }
         var privateKeyObject = this.options.curve.keyFromPrivate(privateKey, 'hex'),
@@ -101,7 +101,7 @@ KeyEncoder.prototype.encodePrivate = function(privateKey, originalFormat, destin
         }
         privateKeyObject = ECPrivateKeyASN.decode(privateKey, 'der')
     } else if (originalFormat === 'pem') {
-        if (!typeof privateKey === 'string') {
+        if (typeof privateKey !== 'string') {
             throw 'private key must be a string'
         }
         privateKeyObject = ECPrivateKeyASN.decode(privateKey, 'pem', this.options.privatePEMOptions)
@@ -126,7 +126,7 @@ KeyEncoder.prototype.encodePublic = function(publicKey, originalFormat, destinat
 
     /* Parse the incoming public key and convert it to a public key object */
     if (originalFormat === 'raw') {
-        if (!typeof publicKey === 'string') {
+        if (typeof publicKey !== 'string') {
             throw 'public key must be a string'
         }
         publicKeyObject = this.publicKeyObject(publicKey)
@@ -140,7 +140,7 @@ KeyEncoder.prototype.encodePublic = function(publicKey, originalFormat, destinat
         }
         publicKeyObject = SubjectPublicKeyInfoASN.decode(publicKey, 'der')
     } else if (originalFormat === 'pem') {
-        if (!typeof publicKey === 'string') {
+        if (typeof publicKey !== 'string') {
             throw 'public key must be a string'
         }
         publicKeyObject = SubjectPublicKeyInfoASN.decode(publicKey, 'pem', this.options.publicPEMOptions)
@@ -160,4 +160,4 @@ KeyEncoder.prototype.encodePublic = function(publicKey, originalFormat, destinat
     }
 }
 
-module.exports = KeyEncoder
\ No newline at end of file
+module.exports = KeyEncoder