diff --git a/disas/riscv.c b/disas/riscv.c
index 4a55348267..b6ced2a26a 100644
--- a/disas/riscv.c
+++ b/disas/riscv.c
@@ -23,7 +23,7 @@
 #include "disas/riscv.h"
 
 typedef enum {
-    rv_op_illegal = 0,
+    /* 0 is reserved for rv_op_illegal. */
     rv_op_lui = 1,
     rv_op_auipc = 2,
     rv_op_jal = 3,
diff --git a/disas/riscv.h b/disas/riscv.h
index 9288255915..debbe69239 100644
--- a/disas/riscv.h
+++ b/disas/riscv.h
@@ -191,6 +191,10 @@ typedef struct {
     const rvc_constraint *constraints;
 } rv_comp_data;
 
+enum {
+    rv_op_illegal = 0
+};
+
 enum {
     rvcd_imm_nz = 0x1
 };