diff --git a/src/Makefile b/src/Makefile
index d089ae27c..2f7370663 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -10,7 +10,7 @@ endif
 
 ifeq ($(debug),yes)
     CPPFLAGS += -DKAK_DEBUG
-    CXXFLAGS + -Og
+    CXXFLAGS += -Og
     suffix := .debug
 else
     ifeq ($(debug),no)