diff --git a/Lib/string.py b/Lib/string.py
index ef0334c..0f4ede2 100644
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -236,12 +236,14 @@
 
     def convert_field(self, value, conversion):
         # do any conversion on the resulting object
-        if conversion == 'r':
-            return repr(value)
+        if conversion is None:
+            return value
         elif conversion == 's':
             return str(value)
-        elif conversion is None:
-            return value
+        elif conversion == 'r':
+            return repr(value)
+        elif conversion == 'a':
+            return ascii(value)
         raise ValueError("Unknown conversion specifier {0!s}".format(conversion))