diff --git a/src/return_types.cpp b/src/return_types.cpp
index e84087e..6e4ac68 100644
--- a/src/return_types.cpp
+++ b/src/return_types.cpp
@@ -18,7 +18,7 @@
 
 namespace Mastodon
 {
-    return_base::operator bool()
+    return_base::operator bool() const
     {
         if (error_code == 0)
         {
@@ -30,7 +30,7 @@ namespace Mastodon
         }
     }
 
-    return_base::operator uint8_t()
+    return_base::operator uint8_t() const
     {
         return error_code;
     }
diff --git a/src/return_types.hpp b/src/return_types.hpp
index f9c324a..1a25c9e 100644
--- a/src/return_types.hpp
+++ b/src/return_types.hpp
@@ -59,14 +59,14 @@ namespace Mastodon
          *
          *  @since  0.100.0
          */
-        operator bool();
+        operator bool() const;
 
         /*!
          *  @brief  Same as return_base::error_code.
          *
          *  @since  0.100.0
          */
-        operator uint8_t();
+        operator uint8_t() const;
     } return_base;
 
     /*!