diff --git a/include/fmt/format.h b/include/fmt/format.h
index 06ecf557..76e4fac6 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -962,8 +962,9 @@ template <typename T = void> struct null {};
 // Workaround an array initialization issue in gcc 4.8.
 template <typename Char> struct fill_t {
  private:
-  enum { max_size = 5 };
-  Char data_[max_size + 1];
+  enum { max_size = 4 };
+  Char data_[max_size];
+  unsigned char size_;
 
  public:
   FMT_CONSTEXPR void operator=(basic_string_view<Char> s) {
@@ -973,15 +974,10 @@ template <typename Char> struct fill_t {
       return;
     }
     for (size_t i = 0; i < size; ++i) data_[i] = s[i];
-    data_[size] = Char();
-  }
-
-  size_t size() const {
-    size_t i = 1;
-    while (data_[i] && i <= max_size) ++i;
-    return i;
+    size_ = static_cast<unsigned char>(size);
   }
 
+  size_t size() const { return size_; }
   const Char* data() const { return data_; }
 
   FMT_CONSTEXPR Char& operator[](size_t index) { return data_[index]; }
@@ -992,6 +988,7 @@ template <typename Char> struct fill_t {
   static FMT_CONSTEXPR fill_t<Char> make() {
     auto fill = fill_t<Char>();
     fill[0] = Char(' ');
+    fill.size_ = 1;
     return fill;
   }
 };
diff --git a/test/format-test.cc b/test/format-test.cc
index 015a7438..8bce69be 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -816,6 +816,8 @@ TEST(FormatterTest, Fill) {
   EXPECT_EQ("foo=", format("{:}=", "foo"));
   EXPECT_EQ(std::string("\0\0\0*", 4), format(string_view("{:\0>4}", 6), '*'));
   EXPECT_EQ("жж42", format("{0:ж>4}", 42));
+  EXPECT_THROW_MSG(format("{:\x80\x80\x80\x80\x80>}", 0), format_error,
+                   "invalid fill");
 }
 
 TEST(FormatterTest, PlusSign) {