diff --git a/librz/crypto/p/crypto_sm4_ecb.c b/librz/crypto/p/crypto_sm4_ecb.c
index 7b1b2fbebc0..f85e4ad8beb 100644
--- a/librz/crypto/p/crypto_sm4_ecb.c
+++ b/librz/crypto/p/crypto_sm4_ecb.c
@@ -185,7 +185,7 @@ static bool update(RzCrypto *cry, const ut8 *buf, int len) {
 	sm4_ecb_encrypt(state->round_keys, len, padded_input, output);
 
 	if (cry->dir == RZ_CRYPTO_DIR_DECRYPT) {
-		while (output[len - 1] == 0x0) {
+		while (len>1 && output[len - 1] == 0x0) {
 			len--;
 		}
 	}