diff --git a/decode.c b/decode.c
index 6d63403..91b09f4 100644
--- a/decode.c
+++ b/decode.c
@@ -131,6 +131,10 @@ decode_prefixes(const uint8_t* buffer, int len, DecodeMode mode,
             if (mode == DECODE_32 && (byte & 0xc0) != 0xc0)
                 goto out;
 
+            // VEX + REX/66/F2/F3/LOCK will #UD.
+            if (prefixes & (PREFIX_REX|PREFIX_REP|PREFIX_REPNZ|PREFIX_LOCK))
+                return -1;
+
             prefixes |= PREFIX_VEX;
             prefixes |= byte & 0x80 ? 0 : PREFIX_REXR;
             if (prefix == 0xc4) // 3-byte VEX