diff --git a/pyupgrade.py b/pyupgrade.py
index 486a440..b9cb902 100644
--- a/pyupgrade.py
+++ b/pyupgrade.py
@@ -818,10 +818,12 @@ def _to_fstring(src, call):
         params[kwd.arg] = _unparse(kwd.value)
 
     parts = []
-    for i, (s, name, spec, conv) in enumerate(parse_format('f' + src)):
+    i = 0
+    for s, name, spec, conv in parse_format('f' + src):
         if name is not None:
             k, dot, rest = name.partition('.')
             name = ''.join((params[k or str(i)], dot, rest))
+            i += 1
         parts.append((s, name, spec, conv))
     return unparse_parsed_string(parts)
 

diff --git a/tests/pyupgrade_test.py b/tests/pyupgrade_test.py
index 80d0b3b..24b47f4 100644
--- a/tests/pyupgrade_test.py
+++ b/tests/pyupgrade_test.py
@@ -721,6 +721,7 @@ def test_fix_fstrings_noop(s):
         ('"{.x} {.y}".format(a, b)', 'f"{a.x} {b.y}"'),
         ('"{} {}".format(a.b, c.d)', 'f"{a.b} {c.d}"'),
         ('"hello {}!".format(name)', 'f"hello {name}!"'),
+        ('"{}{{}}{}".format(escaped, y)', 'f"{escaped}{{}}{y}"'),
 
         # TODO: poor man's f-strings?
         # '"{foo}".format(**locals())'

KeyError running with --py36-plus on matplotlib's ticker.py
Running pyupgrade 1.5 on matplotlib's ticker.py as of, say, mpl3.0rc2 (https://github.com/matplotlib/matplotlib/blob/v3.0.0rc2/lib/matplotlib/ticker.py) gives me
```
Traceback (most recent call last):
  File "/usr/bin/pyupgrade", line 11, in <module>
    sys.exit(main())
  File "/usr/lib/python3.7/site-packages/pyupgrade.py", line 907, in main
    ret |= fix_file(filename, args)
  File "/usr/lib/python3.7/site-packages/pyupgrade.py", line 884, in fix_file
    contents_text = _fix_fstrings(contents_text)
  File "/usr/lib/python3.7/site-packages/pyupgrade.py", line 858, in _fix_fstrings
    tokens[i] = token._replace(src=_to_fstring(token.src, node))
  File "/usr/lib/python3.7/site-packages/pyupgrade.py", line 824, in _to_fstring
    name = ''.join((params[k or str(i)], dot, rest))
KeyError: '3'
```
tests/pyupgrade_test.py::test_fix_fstrings["{}{{}}{}".format(escaped,
tests/pyupgrade_test.py::test_roundtrip_text[] tests/pyupgrade_test.py::test_roundtrip_text[foo] tests/pyupgrade_test.py::test_roundtrip_text[{}] tests/pyupgrade_test.py::test_roundtrip_text[{0}] tests/pyupgrade_test.py::test_roundtrip_text[{named}] tests/pyupgrade_test.py::test_roundtrip_text[{!r}] tests/pyupgrade_test.py::test_roundtrip_text[{:>5}] tests/pyupgrade_test.py::test_roundtrip_text[{{] tests/pyupgrade_test.py::test_roundtrip_text[}}] tests/pyupgrade_test.py::test_roundtrip_text[{0!s:15}] tests/pyupgrade_test.py::test_intentionally_not_round_trip[{:}-{}] tests/pyupgrade_test.py::test_intentionally_not_round_trip[{0:}-{0}] tests/pyupgrade_test.py::test_intentionally_not_round_trip[{0!r:}-{0!r}] tests/pyupgrade_test.py::test_sets[set()-set()] tests/pyupgrade_test.py::test_sets[set((\n))-set((\n))] tests/pyupgrade_test.py::test_sets[set tests/pyupgrade_test.py::test_sets[set(())-set()] tests/pyupgrade_test.py::test_sets[set([])-set()] tests/pyupgrade_test.py::test_sets[set(( tests/pyupgrade_test.py::test_sets[set([1, tests/pyupgrade_test.py::test_sets[set(x tests/pyupgrade_test.py::test_sets[set([(1, tests/pyupgrade_test.py::test_sets[set([((1, tests/pyupgrade_test.py::test_dictcomps[x tests/pyupgrade_test.py::test_dictcomps[dict()-dict()] tests/pyupgrade_test.py::test_dictcomps[(-(] tests/pyupgrade_test.py::test_dictcomps[dict tests/pyupgrade_test.py::test_format_literals['{}'.format(1)-'{}'.format(1)] tests/pyupgrade_test.py::test_format_literals['{'.format(1)-'{'.format(1)] tests/pyupgrade_test.py::test_format_literals['}'.format(1)-'}'.format(1)] tests/pyupgrade_test.py::test_format_literals[x tests/pyupgrade_test.py::test_format_literals['{0}'.format(1)-'{}'.format(1)] tests/pyupgrade_test.py::test_format_literals['''{0}\n{1}\n'''.format(1, tests/pyupgrade_test.py::test_format_literals['{0}' tests/pyupgrade_test.py::test_format_literals[print(\n tests/pyupgrade_test.py::test_format_literals['{0:<{1}}'.format(1, tests/pyupgrade_test.py::test_imports_unicode_literals[import tests/pyupgrade_test.py::test_imports_unicode_literals[from tests/pyupgrade_test.py::test_imports_unicode_literals[x tests/pyupgrade_test.py::test_imports_unicode_literals["""docstring"""\nfrom tests/pyupgrade_test.py::test_unicode_literals[(-False-(] tests/pyupgrade_test.py::test_unicode_literals[u''-False-u''] tests/pyupgrade_test.py::test_unicode_literals[u''-True-''] tests/pyupgrade_test.py::test_unicode_literals[from tests/pyupgrade_test.py::test_unicode_literals["""with tests/pyupgrade_test.py::test_noop_octal_literals[0-0] tests/pyupgrade_test.py::test_noop_octal_literals[00-00] tests/pyupgrade_test.py::test_noop_octal_literals[1-1] tests/pyupgrade_test.py::test_noop_octal_literals[12345-12345] tests/pyupgrade_test.py::test_noop_octal_literals[1.2345-1.2345] tests/pyupgrade_test.py::test_is_bytestring_true[b''] tests/pyupgrade_test.py::test_is_bytestring_true[b""] tests/pyupgrade_test.py::test_is_bytestring_true[B""] tests/pyupgrade_test.py::test_is_bytestring_true[B''] tests/pyupgrade_test.py::test_is_bytestring_true[rb''0] tests/pyupgrade_test.py::test_is_bytestring_true[rb''1] tests/pyupgrade_test.py::test_is_bytestring_false[] tests/pyupgrade_test.py::test_is_bytestring_false[""] tests/pyupgrade_test.py::test_is_bytestring_false[''] tests/pyupgrade_test.py::test_is_bytestring_false[u""] tests/pyupgrade_test.py::test_is_bytestring_false["b"] tests/pyupgrade_test.py::test_parse_percent_format[""-expected0] tests/pyupgrade_test.py::test_parse_percent_format["%%"-expected1] tests/pyupgrade_test.py::test_parse_percent_format["%s"-expected2] tests/pyupgrade_test.py::test_parse_percent_format["%s tests/pyupgrade_test.py::test_parse_percent_format["%(hi)s"-expected4] tests/pyupgrade_test.py::test_parse_percent_format["%()s"-expected5] tests/pyupgrade_test.py::test_parse_percent_format["%#o"-expected6] tests/pyupgrade_test.py::test_parse_percent_format["% tests/pyupgrade_test.py::test_parse_percent_format["%5d"-expected8] tests/pyupgrade_test.py::test_parse_percent_format["%*d"-expected9] tests/pyupgrade_test.py::test_parse_percent_format["%.f"-expected10] tests/pyupgrade_test.py::test_parse_percent_format["%.5f"-expected11] tests/pyupgrade_test.py::test_parse_percent_format["%.*f"-expected12] tests/pyupgrade_test.py::test_parse_percent_format["%ld"-expected13] tests/pyupgrade_test.py::test_parse_percent_format["%(complete)#4.4f"-expected14] tests/pyupgrade_test.py::test_percent_to_format[%s-{}] tests/pyupgrade_test.py::test_percent_to_format[%%%s-%{}] tests/pyupgrade_test.py::test_percent_to_format[%(foo)s-{foo}] tests/pyupgrade_test.py::test_percent_to_format[%2f-{:2f}] tests/pyupgrade_test.py::test_percent_to_format[%r-{!r}] tests/pyupgrade_test.py::test_percent_to_format[%a-{!a}] tests/pyupgrade_test.py::test_simplify_conversion_flag[-] tests/pyupgrade_test.py::test_simplify_conversion_flag[ tests/pyupgrade_test.py::test_simplify_conversion_flag[#0- tests/pyupgrade_test.py::test_simplify_conversion_flag[--<] tests/pyupgrade_test.py::test_percent_format_noop["%s" tests/pyupgrade_test.py::test_percent_format_noop[b"%s" tests/pyupgrade_test.py::test_percent_format_noop["%*s" tests/pyupgrade_test.py::test_percent_format_noop["%.*s" tests/pyupgrade_test.py::test_percent_format_noop["%d" tests/pyupgrade_test.py::test_percent_format_noop["%i" tests/pyupgrade_test.py::test_percent_format_noop["%u" tests/pyupgrade_test.py::test_percent_format_noop["%c" tests/pyupgrade_test.py::test_percent_format_noop["%#o" tests/pyupgrade_test.py::test_percent_format_noop["%()s" tests/pyupgrade_test.py::test_percent_format_noop["%4%" tests/pyupgrade_test.py::test_percent_format_noop["%.2r" tests/pyupgrade_test.py::test_percent_format_noop["%.2a" tests/pyupgrade_test.py::test_percent_format_noop[i tests/pyupgrade_test.py::test_percent_format_noop["%(1)s" tests/pyupgrade_test.py::test_percent_format_noop["%(a)s" tests/pyupgrade_test.py::test_percent_format_noop["%(ab)s" tests/pyupgrade_test.py::test_percent_format["trivial" tests/pyupgrade_test.py::test_percent_format["%s%% tests/pyupgrade_test.py::test_percent_format["%3f" tests/pyupgrade_test.py::test_percent_format["%-5s" tests/pyupgrade_test.py::test_percent_format["brace tests/pyupgrade_test.py::test_percent_format["%(k)s" tests/pyupgrade_test.py::test_fix_super_noop[x(] tests/pyupgrade_test.py::test_fix_super_noop[class tests/pyupgrade_test.py::test_fix_super_noop[def tests/pyupgrade_test.py::test_fix_super[class tests/pyupgrade_test.py::test_fix_fstrings_noop[(] tests/pyupgrade_test.py::test_fix_fstrings_noop["{}" tests/pyupgrade_test.py::test_fix_fstrings_noop["{}".format(\n tests/pyupgrade_test.py::test_fix_fstrings_noop["{} tests/pyupgrade_test.py::test_fix_fstrings_noop["{foo} tests/pyupgrade_test.py::test_fix_fstrings_noop["{0} tests/pyupgrade_test.py::test_fix_fstrings_noop["{x} tests/pyupgrade_test.py::test_fix_fstrings_noop["{x.y} tests/pyupgrade_test.py::test_fix_fstrings_noop[b"{} tests/pyupgrade_test.py::test_fix_fstrings_noop["{:{}}".format(x, tests/pyupgrade_test.py::test_fix_fstrings["{} tests/pyupgrade_test.py::test_fix_fstrings["{1} tests/pyupgrade_test.py::test_fix_fstrings["{x.y}".format(x=z)-f"{z.y}"] tests/pyupgrade_test.py::test_fix_fstrings["{.x} tests/pyupgrade_test.py::test_fix_fstrings["hello tests/pyupgrade_test.py::test_main_trivial tests/pyupgrade_test.py::test_main_noop tests/pyupgrade_test.py::test_main_syntax_error tests/pyupgrade_test.py::test_main_non_utf8_bytes tests/pyupgrade_test.py::test_py3_plus_argument_unicode_literals tests/pyupgrade_test.py::test_py3_plus_super tests/pyupgrade_test.py::test_py36_plus_fstrings