diff --git a/src/mastodon-cpp.cpp b/src/mastodon-cpp.cpp
index f3c6c66..a9c1356 100644
--- a/src/mastodon-cpp.cpp
+++ b/src/mastodon-cpp.cpp
@@ -26,8 +26,7 @@
 #include <iostream>
 
 using namespace Mastodon;
-
-constexpr return_base::operator const bool() const
+return_base::operator const bool() const
 {
     if (error_code == 0)
     {
@@ -39,12 +38,12 @@ constexpr return_base::operator const bool() const
     }
 }
 
-constexpr return_base::operator const uint8_t() const
+return_base::operator const uint8_t() const
 {
     return error_code;
 }
 
-const return_call::operator const string() const
+return_call::operator const string() const
 {
     return answer;
 }
diff --git a/src/mastodon-cpp.hpp b/src/mastodon-cpp.hpp
index 0a55e04..6cdaf51 100644
--- a/src/mastodon-cpp.hpp
+++ b/src/mastodon-cpp.hpp
@@ -47,8 +47,8 @@ namespace Mastodon
         uint8_t error_code = 0;     // NOTE: http://mazack.org/unix/errno.php
         string error_message;
 
-        constexpr operator const bool() const;
-        constexpr operator const uint8_t() const;
+        operator const bool() const;
+        operator const uint8_t() const;
     } return_base;
 
     /*!
@@ -60,7 +60,7 @@ namespace Mastodon
         uint16_t http_error_code = 0;
         string answer;
 
-        const operator const string() const;
+        operator const string() const;
         friend std::ostream &operator <<(std::ostream &out,
                                          const return_call &ret);