diff --git a/include/promise-cpp/promise.hpp b/include/promise-cpp/promise.hpp
--- a/include/promise-cpp/promise.hpp
+++ b/include/promise-cpp/promise.hpp
@@ -2,7 +2,7 @@
 #ifndef INC_PROMISE_HPP_
 #define INC_PROMISE_HPP_
 
-#ifdef PROMISE_HEADER_ONLY
+#ifdef PROMISE_HEADONLY
 #define PROMISE_API inline
 #elif defined PROMISE_BUILD_SHARED