diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index da176b8..f068ca6 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -1463,6 +1463,27 @@
         self.assertEqual(u'{0:10000}'.format(u''), u' ' * 10000)
         self.assertEqual(u'{0:10000000}'.format(u''), u' ' * 10000000)
 
+        # issue 12546: use \x00 as a fill character
+        self.assertEqual('{0:\x00<6s}'.format('foo'), 'foo\x00\x00\x00')
+        self.assertEqual('{0:\x01<6s}'.format('foo'), 'foo\x01\x01\x01')
+        self.assertEqual('{0:\x00^6s}'.format('foo'), '\x00foo\x00\x00')
+        self.assertEqual('{0:^6s}'.format('foo'), ' foo  ')
+
+        self.assertEqual('{0:\x00<6}'.format(3), '3\x00\x00\x00\x00\x00')
+        self.assertEqual('{0:\x01<6}'.format(3), '3\x01\x01\x01\x01\x01')
+        self.assertEqual('{0:\x00^6}'.format(3), '\x00\x003\x00\x00\x00')
+        self.assertEqual('{0:<6}'.format(3), '3     ')
+
+        self.assertEqual('{0:\x00<6}'.format(3.14), '3.14\x00\x00')
+        self.assertEqual('{0:\x01<6}'.format(3.14), '3.14\x01\x01')
+        self.assertEqual('{0:\x00^6}'.format(3.14), '\x003.14\x00')
+        self.assertEqual('{0:^6}'.format(3.14), ' 3.14 ')
+
+        self.assertEqual('{0:\x00<12}'.format(3+2.0j), '(3+2j)\x00\x00\x00\x00\x00\x00')
+        self.assertEqual('{0:\x01<12}'.format(3+2.0j), '(3+2j)\x01\x01\x01\x01\x01\x01')
+        self.assertEqual('{0:\x00^12}'.format(3+2.0j), '\x00\x00\x00(3+2j)\x00\x00\x00')
+        self.assertEqual('{0:^12}'.format(3+2.0j), '   (3+2j)   ')
+
         # format specifiers for user defined type
         self.assertEqual(u'{0:abc}'.format(C()), u'abc')
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 79eaada..f6a2c65 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -40,6 +40,9 @@
 - Issue #19638: Fix possible crash / undefined behaviour from huge (more than 2
   billion characters) input strings in _Py_dg_strtod.
 
+- Issue #12546: Allow \x00 to be used as a fill character when using str, int,
+  float, and complex __format__ methods.
+
 Library
 -------
 
diff --git a/Objects/stringlib/formatter.h b/Objects/stringlib/formatter.h
index fd22751..122abe6 100644
--- a/Objects/stringlib/formatter.h
+++ b/Objects/stringlib/formatter.h
@@ -180,8 +180,9 @@
 
     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';
@@ -195,6 +196,7 @@
     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;
     }
@@ -218,7 +220,7 @@
     }
 
     /* 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 = '=';
@@ -715,8 +717,7 @@
 
     /* Write into that space. First the padding. */
     p = fill_padding(STRINGLIB_STR(result), len,
-                     format->fill_char=='\0'?' ':format->fill_char,
-                     lpad, rpad);
+                     format->fill_char, lpad, rpad);
 
     /* Then the source string. */
     memcpy(p, STRINGLIB_STR(value), len * sizeof(STRINGLIB_CHAR));
@@ -893,8 +894,7 @@
 
     /* Populate the memory. */
     fill_number(STRINGLIB_STR(result), &spec, pnumeric_chars, n_digits,
-                prefix, format->fill_char == '\0' ? ' ' : format->fill_char,
-                &locale, format->type == 'X');
+                prefix, format->fill_char, &locale, format->type == 'X');
 
 done:
     Py_XDECREF(tmp);
@@ -1048,8 +1048,7 @@
 
     /* Populate the memory. */
     fill_number(STRINGLIB_STR(result), &spec, p, n_digits, NULL,
-                format->fill_char == '\0' ? ' ' : format->fill_char, &locale,
-                0);
+                format->fill_char, &locale, 0);
 
 done:
     PyMem_Free(buf);
@@ -1265,8 +1264,7 @@
     /* Populate the memory. First, the padding. */
     p = fill_padding(STRINGLIB_STR(result),
                      n_re_total + n_im_total + 1 + add_parens * 2,
-                     format->fill_char=='\0' ? ' ' : format->fill_char,
-                     lpad, rpad);
+                     format->fill_char, lpad, rpad);
 
     if (add_parens)
         *p++ = '(';