diff --git a/format.cc b/format.cc
index 92e323aa..ceadbce8 100644
--- a/format.cc
+++ b/format.cc
@@ -753,27 +753,24 @@ void fmt::BasicWriter<Char>::write_str(
 template <typename Char>
 inline const Arg
     &fmt::BasicFormatter<Char>::parse_arg_index(const Char *&s) {
-  unsigned arg_index = 0;
+  const Arg *arg = 0;
   if (*s < '0' || *s > '9') {
-    if (*s != '}' && *s != ':')
-      throw FormatError("invalid format string");
-    const Arg &arg = next_arg();
-    if (error_)
-      throw FormatError(error_);
-    return arg;
+    arg = &next_arg();
+  } else {
+    if (next_arg_index_ > 0)
+      error_ = "cannot switch from automatic to manual argument indexing";
+    next_arg_index_ = -1;
+    unsigned arg_index = parse_nonnegative_int(s, error_);
+    if (arg_index < args_.size())
+      arg = &args_[arg_index];
+    else if (!error_)
+        error_ = "argument index is out of range in format";
   }
-  if (next_arg_index_ > 0) {
+  if (error_) {
     throw FormatError(
-        "cannot switch from automatic to manual argument indexing");
+          *s != '}' && *s != ':' ? "invalid format string" : error_);
   }
-  next_arg_index_ = -1;
-  arg_index = parse_nonnegative_int(s, error_);
-  if (arg_index >= args_.size()) {
-    if (!error_)
-      error_ = "argument index is out of range in format";
-    return DUMMY_ARG;
-  }
-  return args_[arg_index];
+  return *arg;
 }
 
 template <typename Char>
@@ -1198,8 +1195,6 @@ const Char *fmt::BasicFormatter<Char>::format(
 
   if (*s++ != '}')
     throw FormatError("unmatched '{' in format");
-  if (error_)
-    throw FormatError(error_);
   start_ = s;
 
   // Format argument.
diff --git a/test/format-test.cc b/test/format-test.cc
index bbe51da0..0fbff0bb 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -587,7 +587,7 @@ TEST(FormatterTest, Escape) {
 TEST(FormatterTest, UnmatchedBraces) {
   EXPECT_THROW_MSG(format("{"), FormatError, "invalid format string");
   EXPECT_THROW_MSG(format("}"), FormatError, "unmatched '}' in format");
-  EXPECT_THROW_MSG(format("{0{}"), FormatError, "unmatched '{' in format");
+  EXPECT_THROW_MSG(format("{0{}"), FormatError, "invalid format string");
 }
 
 TEST(FormatterTest, NoArgs) {
@@ -607,19 +607,19 @@ TEST(FormatterTest, ArgsInDifferentPositions) {
 TEST(FormatterTest, ArgErrors) {
   EXPECT_THROW_MSG(format("{"), FormatError, "invalid format string");
   EXPECT_THROW_MSG(format("{x}"), FormatError, "invalid format string");
-  EXPECT_THROW_MSG(format("{0"), FormatError, "unmatched '{' in format");
+  EXPECT_THROW_MSG(format("{0"), FormatError, "invalid format string");
   EXPECT_THROW_MSG(format("{0}"), FormatError,
       "argument index is out of range in format");
 
   char format_str[BUFFER_SIZE];
   safe_sprintf(format_str, "{%u", INT_MAX);
-  EXPECT_THROW_MSG(format(format_str), FormatError, "unmatched '{' in format");
+  EXPECT_THROW_MSG(format(format_str), FormatError, "invalid format string");
   safe_sprintf(format_str, "{%u}", INT_MAX);
   EXPECT_THROW_MSG(format(format_str), FormatError,
       "argument index is out of range in format");
 
   safe_sprintf(format_str, "{%u", INT_MAX + 1u);
-  EXPECT_THROW_MSG(format(format_str), FormatError, "unmatched '{' in format");
+  EXPECT_THROW_MSG(format(format_str), FormatError, "invalid format string");
   safe_sprintf(format_str, "{%u}", INT_MAX + 1u);
   EXPECT_THROW_MSG(format(format_str), FormatError,
                    "number is too big in format");
@@ -896,7 +896,7 @@ TEST(FormatterTest, Width) {
   char format_str[BUFFER_SIZE];
   safe_sprintf(format_str, "{0:%u", UINT_MAX);
   increment(format_str + 3);
-  EXPECT_THROW_MSG(format(format_str), FormatError,
+  EXPECT_THROW_MSG(format(format_str, 0), FormatError,
                    "number is too big in format");
   std::size_t size = std::strlen(format_str);
   format_str[size] = '}';
@@ -905,7 +905,7 @@ TEST(FormatterTest, Width) {
                    "number is too big in format");
 
   safe_sprintf(format_str, "{0:%u", INT_MAX + 1u);
-  EXPECT_THROW_MSG(format(format_str), FormatError,
+  EXPECT_THROW_MSG(format(format_str, 0), FormatError,
                    "number is too big in format");
   safe_sprintf(format_str, "{0:%u}", INT_MAX + 1u);
   EXPECT_THROW_MSG(format(format_str, 0), FormatError,
@@ -928,7 +928,7 @@ TEST(FormatterTest, Precision) {
   char format_str[BUFFER_SIZE];
   safe_sprintf(format_str, "{0:.%u", UINT_MAX);
   increment(format_str + 4);
-  EXPECT_THROW_MSG(format(format_str), FormatError,
+  EXPECT_THROW_MSG(format(format_str, 0), FormatError,
                    "number is too big in format");
   std::size_t size = std::strlen(format_str);
   format_str[size] = '}';
@@ -937,7 +937,7 @@ TEST(FormatterTest, Precision) {
                    "number is too big in format");
 
   safe_sprintf(format_str, "{0:.%u", INT_MAX + 1u);
-  EXPECT_THROW_MSG(format(format_str), FormatError,
+  EXPECT_THROW_MSG(format(format_str, 0), FormatError,
                    "number is too big in format");
   safe_sprintf(format_str, "{0:.%u}", INT_MAX + 1u);
   EXPECT_THROW_MSG(format(format_str, 0), FormatError,
@@ -1002,7 +1002,7 @@ TEST(FormatterTest, RuntimePrecision) {
   char format_str[BUFFER_SIZE];
   safe_sprintf(format_str, "{0:.{%u", UINT_MAX);
   increment(format_str + 5);
-  EXPECT_THROW_MSG(format(format_str), FormatError, "unmatched '{' in format");
+  EXPECT_THROW_MSG(format(format_str, 0), FormatError, "invalid format string");
   std::size_t size = std::strlen(format_str);
   format_str[size] = '}';
   format_str[size + 1] = 0;