diff --git a/src/base58.coffee b/src/base58.coffee
index eadb6cd..6fff825 100644
--- a/src/base58.coffee
+++ b/src/base58.coffee
@@ -16,7 +16,7 @@ class Base58Builder
     num = 0
     for char, index in str.split(//).reverse()
       if (char_index = @alphabet.indexOf(char)) == -1
-        throw new Error('Value passed not a valid Base58 string.')
+        throw new Error('Value passed is not a valid Base58 string.')
       num += char_index * Math.pow(@base, index)
     num
 
diff --git a/test/base58_test.coffee b/test/base58_test.coffee
index 0dad57f..3af1d9a 100644
--- a/test/base58_test.coffee
+++ b/test/base58_test.coffee
@@ -28,4 +28,4 @@ describe 'Base58', ->
     describe 'when passed a non-Base58 string', ->
       it 'throws an error', ->
         (-> Base58.decode('>_<')).should
-          .throw('Value passed not a valid Base58 string.')
\ No newline at end of file
+          .throw('Value passed is not a valid Base58 string.')