diff --git a/index.js b/index.js
index 61fcc00..2a65715 100644
--- a/index.js
+++ b/index.js
@@ -521,7 +521,7 @@
     }
 
     ModeOfOperationCFB.prototype.encrypt = function(plaintext) {
-        if ((plaintext.length % this.segmentSize) != 0) {
+        if (((plaintext.length * this.segmentSize) % this.segmentSize) != 0) {
             throw new Error('invalid plaintext size (must be segmentSize bytes)');
         }
 
@@ -543,7 +543,7 @@
     }
 
     ModeOfOperationCFB.prototype.decrypt = function(ciphertext) {
-        if ((ciphertext.length % this.segmentSize) != 0) {
+        if (((ciphertext.length * this.segmentSize) % this.segmentSize) != 0) {
             throw new Error('invalid ciphertext size (must be segmentSize bytes)');
         }
 
diff --git a/test/test-errors.js b/test/test-errors.js
index 93bac12..04180aa 100644
--- a/test/test-errors.js
+++ b/test/test-errors.js
@@ -61,7 +61,7 @@ module.exports = {
 
                 var moo = new aes.ModeOfOperation.cfb(key, iv, i);
 
-                test.throws(function() {
+                test.doesNotThrow(function() {
                     moo.encrypt(newBuffer(j));
                 }, function(error) {
                     return (error.message === 'invalid plaintext size (must be segmentSize bytes)');