diff --git a/nemu/tests/Makefile b/nemu/tests/Makefile
index 7709a27..6804b41 100644
--- a/nemu/tests/Makefile
+++ b/nemu/tests/Makefile
@@ -1,4 +1,5 @@
 TEST_SRCS += tests/expr_test.c
+YACC = bison
 
 $(OBJ_DIR)/%: %.c $(TEST_OBJS) app
 	@mkdir -p $(dir $@)