diff --git a/format.cc b/format.cc
index 673cc2a8..bf355910 100644
--- a/format.cc
+++ b/format.cc
@@ -175,19 +175,6 @@ int parse_nonnegative_int(const Char *&s) {
   return value;
 }
 
-template <typename Char>
-const Char *find_closing_brace(const Char *s, int num_open_braces = 1) {
-  for (int n = num_open_braces; *s; ++s) {
-    if (*s == '{') {
-      ++n;
-    } else if (*s == '}') {
-      if (--n == 0)
-        return s;
-    }
-  }
-  throw fmt::FormatError("unmatched '{' in format");
-}
-
 template <typename Char>
 void check_sign(const Char *&s, const Arg &arg) {
   char sign = static_cast<char>(*s);
@@ -574,7 +561,7 @@ class fmt::internal::ArgFormatter :
   }
 
   void visit_custom(Arg::CustomValue c) {
-    c.format(&formatter_, c.value, format_);
+    c.format(&formatter_, c.value, &format_);
   }
 };
 
@@ -1019,11 +1006,13 @@ void fmt::internal::PrintfFormatter<Char>::format(
       spec.type_ = 'x';
       writer.write_int(reinterpret_cast<uintptr_t>(arg.pointer_value), spec);
       break;
-    case Arg::CUSTOM:
+    case Arg::CUSTOM: {
       if (spec.type_)
         internal::report_unknown_type(spec.type_, "object");
-      arg.custom.format(&writer, arg.custom.value, "s");
+      const void *s = "s";
+      arg.custom.format(&writer, arg.custom.value, &s);
       break;
+    }
     default:
       assert(false);
       break;
@@ -1034,14 +1023,14 @@ void fmt::internal::PrintfFormatter<Char>::format(
 
 template <typename Char>
 const Char *fmt::BasicFormatter<Char>::format(
-    const Char *format_str, const Arg &arg) {
+    const Char *&format_str, const Arg &arg) {
   const Char *s = format_str;
   const char *error = 0;
   FormatSpec spec;
   if (*s == ':') {
     if (arg.type == Arg::CUSTOM) {
-      arg.custom.format(this, arg.custom.value, s);
-      return find_closing_brace(s) + 1;
+      arg.custom.format(this, arg.custom.value, &s);
+      return s;
     }
     ++s;
     // Parse fill and alignment.
diff --git a/format.h b/format.h
index 6ee80112..ed681fb6 100644
--- a/format.h
+++ b/format.h
@@ -132,7 +132,7 @@ template <typename Char>
 class BasicFormatter;
 
 template <typename Char, typename T>
-void format(BasicFormatter<Char> &f, const Char *format_str, const T &value);
+void format(BasicFormatter<Char> &f, const Char *&format_str, const T &value);
 
 /**
   \rst
@@ -583,7 +583,7 @@ struct Arg {
   };
 
   typedef void (*FormatFunc)(
-      void *formatter, const void *arg, const void *format_str);
+      void *formatter, const void *arg, void *format_str_ptr);
 
   struct CustomValue {
     const void *value;
@@ -634,9 +634,9 @@ class MakeArg : public Arg {
   // Formats an argument of a custom type, such as a user-defined class.
   template <typename T>
   static void format_custom_arg(
-      void *formatter, const void *arg, const void *format_str) {
+      void *formatter, const void *arg, void *format_str_ptr) {
     format(*static_cast<BasicFormatter<Char>*>(formatter),
-        static_cast<const Char*>(format_str), *static_cast<const T*>(arg));
+        *static_cast<const Char**>(format_str_ptr), *static_cast<const T*>(arg));
   }
 
 public:
@@ -896,7 +896,7 @@ public:
 
   void format(BasicStringRef<Char> format_str, const ArgList &args);
 
-  const Char *format(const Char *format_str, const internal::Arg &arg);
+  const Char *format(const Char *&format_str, const internal::Arg &arg);
 };
 
 enum Alignment {
@@ -1681,10 +1681,10 @@ void BasicWriter<Char>::write_int(T value, const Spec &spec) {
 
 // Formats a value.
 template <typename Char, typename T>
-void format(BasicFormatter<Char> &f, const Char *format_str, const T &value) {
+void format(BasicFormatter<Char> &f, const Char *&format_str, const T &value) {
   std::basic_ostringstream<Char> os;
   os << value;
-  f.format(format_str, internal::MakeArg<Char>(os.str()));
+  format_str = f.format(format_str, internal::MakeArg<Char>(os.str()));
 }
 
 // Reports a system error without throwing an exception.
diff --git a/test/util-test.cc b/test/util-test.cc
index 7f4fd84e..b0b80766 100644
--- a/test/util-test.cc
+++ b/test/util-test.cc
@@ -238,7 +238,8 @@ TEST(ArgTest, MakeArg) {
   EXPECT_EQ(&t, arg.custom.value);
   fmt::Writer w;
   fmt::BasicFormatter<char> formatter(w);
-  arg.custom.format(&formatter, &t, "}");
+  const char *s = "}";
+  arg.custom.format(&formatter, &t, &s);
   EXPECT_EQ("test", w.str());
 }