diff --git a/README b/README
index 3e82e64..a5bd464 100644
--- a/README
+++ b/README
@@ -25,6 +25,10 @@ TESTING
 
 To run pydkim's test suite:
 
+    PYTHONPATH=. python dkim
+or
+    python test.py
+or
     PYTHONPATH=. python -m unittest dkim.tests.test_suite
 
 Alternatively, if you have testrepository installed:
diff --git a/dkim/__init__.py b/dkim/__init__.py
index ff4023f..a7dece9 100644
--- a/dkim/__init__.py
+++ b/dkim/__init__.py
@@ -193,7 +193,14 @@ def rfc822_parse(message):
 
 
 def fold(header):
-    """Fold a header line into multiple crlf-separated lines at column 72."""
+    """Fold a header line into multiple crlf-separated lines at column 72.
+    >>> fold(b'foo')
+    'foo'
+    >>> fold(b'foo'*25).splitlines()[-1]
+    ' foo'
+    >>> len(fold(b'foo'*25).splitlines()[0])
+    72
+    """
     i = header.rfind(b"\r\n ")
     if i == -1:
         pre = b""
@@ -204,10 +211,10 @@ def fold(header):
     while len(header) > 72:
         i = header[:72].rfind(b" ")
         if i == -1:
-            j = i
+            j = 72
         else:
             j = i + 1
-        pre += header[:i] + b"\r\n "
+        pre += header[:j] + b"\r\n "
         header = header[j:]
     return pre + header
 
diff --git a/dkim/__main__.py b/dkim/__main__.py
new file mode 100644
index 0000000..1e0a02f
--- /dev/null
+++ b/dkim/__main__.py
@@ -0,0 +1,7 @@
+import unittest
+import doctest
+import dkim
+from tests import test_suite
+
+doctest.testmod(dkim)
+unittest.TextTestRunner().run(test_suite())