diff --git a/git-compat-util.h b/git-compat-util.h
index 3a7216f..3a051de 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1111,6 +1111,9 @@ static inline int regexec_buf(const regex_t *preg, const char *buf, size_t size,
 #define HAVE_VARIADIC_MACROS 1
 #endif
 
+/* usage.c: only to be used for testing BUG() implementation (see test-tool) */
+extern int BUG_exit_code;
+
 #ifdef HAVE_VARIADIC_MACROS
 __attribute__((format (printf, 3, 4))) NORETURN
 void BUG_fl(const char *file, int line, const char *fmt, ...);