diff --git a/Makefile.am b/Makefile.am
index 3cffd9c2..b7468198 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -261,6 +261,8 @@ tests_test_umockdev_LDADD =	\
 tests_test_ioctl_tree_SOURCES =	\
 	src/ioctl_tree.c	\
 	tests/test-ioctl-tree.c	\
+	src/debug.h		\
+	src/debug.c		\
 	$(NULL)
 
 tests_test_ioctl_tree_CFLAGS =	\
diff --git a/src/debug.h b/src/debug.h
index 0d18fc48..feb7f927 100644
--- a/src/debug.h
+++ b/src/debug.h
@@ -13,7 +13,7 @@
 #define DBG_IOCTL   (1 << 3)
 #define DBG_IOCTL_TREE (1 << 4)
 
-unsigned debug_categories;
+extern unsigned debug_categories;
 
 void init_debug(void) __attribute__((constructor));