diff --git a/internal.h b/internal.h
index b83999464c..c9b16ca49a 100644
--- a/internal.h
+++ b/internal.h
@@ -2050,6 +2050,15 @@ rb_obj_builtin_type(VALUE obj)
 }
 #endif
 
+/* A macro for defining a flexible array, like: VALUE ary[FLEX_ARY_LEN]; */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
+# define FLEX_ARY_LEN   /* VALUE ary[]; */
+#elif defined(__GNUC__) && !defined(__STRICT_ANSI__)
+# define FLEX_ARY_LEN 0 /* VALUE ary[0]; */
+#else
+# define FLEX_ARY_LEN 1 /* VALUE ary[1]; */
+#endif
+
 #if defined(__cplusplus)
 #if 0
 { /* satisfy cc-mode */
diff --git a/iseq.c b/iseq.c
index 5cdac49de5..ee49026c9a 100644
--- a/iseq.c
+++ b/iseq.c
@@ -2836,15 +2836,7 @@ struct succ_index_table {
 	unsigned int rank;
 	uint64_t small_block_ranks; /* 9 bits * 7 = 63 bits */
 	uint64_t bits[512/64];
-    } succ_part[
-#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
-	/* flexible array member */
-#elif defined(__GNUC__) && !defined(__STRICT_ANSI__)
-	0
-#else
-	1
-#endif
-    ];
+    } succ_part[FLEX_ARY_LEN];
 } succ_index_table;
 
 #define imm_block_rank_set(v, i, r) (v) |= (uint64_t)(r) << (7 * (i))