diff -r 47a36d2d2b44 Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py	Tue Jul 12 19:23:43 2011 -0500
+++ b/Lib/test/test_unicode.py	Wed Jul 13 11:06:16 2011 +0200
@@ -584,6 +584,10 @@
         self.assertEqual('{0:10000}'.format(''), ' ' * 10000)
         self.assertEqual('{0:10000000}'.format(''), ' ' * 10000000)
 
+        #issue 12546
+        self.assertEqual('{0:\x00<6s}'.format('foo'), 'foo\x00\x00\x00')
+        self.assertEqual('{0:<6s}'.format('foo'), 'foo   ')
+
         # format specifiers for user defined type
         self.assertEqual('{0:abc}'.format(C()), 'abc')
 
diff -r 47a36d2d2b44 Objects/stringlib/formatter.h
--- a/Objects/stringlib/formatter.h	Tue Jul 12 19:23:43 2011 -0500
+++ b/Objects/stringlib/formatter.h	Wed Jul 13 11:06:16 2011 +0200
@@ -182,8 +182,9 @@
 
     Py_ssize_t consumed;
     int align_specified = 0;
+    int fill_char_specified = 0;
 
-    format->fill_char = '\0';
+    format->fill_char = ' ';
     format->align = default_align;
     format->alternate = 0;
     format->sign = '\0';
@@ -197,6 +198,7 @@
     if (end-ptr >= 2 && is_alignment_token(ptr[1])) {
         format->align = ptr[1];
         format->fill_char = ptr[0];
+        fill_char_specified = 1;
         align_specified = 1;
         ptr += 2;
     }
@@ -220,7 +222,7 @@
     }
 
     /* The special case for 0-padding (backwards compat) */
-    if (format->fill_char == '\0' && end-ptr >= 1 && ptr[0] == '0') {
+    if (!fill_char_specified && end-ptr >= 1 && ptr[0] == '0') {
         format->fill_char = '0';
         if (!align_specified) {
             format->align = '=';
@@ -717,7 +719,7 @@
 
     /* Write into that space. First the padding. */
     p = fill_padding(STRINGLIB_STR(result), len,
-                     format->fill_char=='\0'?' ':format->fill_char,
+                     format->fill_char,
                      lpad, rpad);
 
     /* Then the source string. */