diff --git a/include/spdlog/details/format.h b/include/spdlog/details/format.h
index fd390caa..0e63d8e9 100644
--- a/include/spdlog/details/format.h
+++ b/include/spdlog/details/format.h
@@ -28,7 +28,7 @@
 #ifndef FMT_FORMAT_H_
 #define FMT_FORMAT_H_
 
-#ifdef FMT_HEADER_ONLY
+#define FMT_HEADER_ONLY
 
 #include <stdint.h>
 
@@ -2775,4 +2775,4 @@ FMT_VARIADIC(int, fprintf, std::FILE *, StringRef)
 # include "format.cc"
 #endif
 
-#endif  // FMT_FORMAT_H_
\ No newline at end of file
+#endif  // FMT_FORMAT_H_