diff --git a/tcg/tcg.h b/tcg/tcg.h
index a3fb88cd04bbc10612a56422687601ff8f73da3c..4f4b1fa4d2ccdc55ce3f1c120585044271464d29 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -147,9 +147,8 @@ typedef enum TCGOpcode {
 #define tcg_regset_not(d, a) (d) = ~(a)
 
 #ifndef TCG_TARGET_INSN_UNIT_SIZE
-#define TCG_TARGET_INSN_UNIT_SIZE 1
-#endif
-#if TCG_TARGET_INSN_UNIT_SIZE == 1
+# error "Missing TCG_TARGET_INSN_UNIT_SIZE"
+#elif TCG_TARGET_INSN_UNIT_SIZE == 1
 typedef uint8_t tcg_insn_unit;
 #elif TCG_TARGET_INSN_UNIT_SIZE == 2
 typedef uint16_t tcg_insn_unit;