diff --git a/inc/format.h b/inc/format.h
index 0446e90..814607d 100644
--- a/inc/format.h
+++ b/inc/format.h
@@ -103,7 +103,7 @@ constexpr char_array_t format_args(char_array_t result, first_arg_t first_arg,
 
 
 template <detail::ConstString s, typename... args_t>
-std::array<char, detail::get_output_len<s>()> format(args_t... args) {
+constexpr std::array<char, detail::get_output_len<s>()> format(args_t... args) {
     constexpr auto parse_result = detail::parse_string<s>();
     static_assert(parse_result.is_valid, "Syntax error in format string");
 
diff --git a/src/main.cpp b/src/main.cpp
index 5fb0e56..a6c2d26 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -12,7 +12,7 @@ public:
 
 
 int main() {
-    const auto formatted = format<"Test: {:3.8f} {:5.5} {:05.2}">(3.4, "abc", 1234);
+    constexpr auto formatted = format<"Test: {:3.8f} {:5.5} {:05.2}">(3.4, "abc", 1234);
 
     for (const auto& c : formatted)
         std::cout << c;