diff --git a/client/emv/emv_roca.c b/client/emv/emv_roca.c
index 2843cb2c1..48845c99f 100644
--- a/client/emv/emv_roca.c
+++ b/client/emv/emv_roca.c
@@ -66,7 +66,7 @@ int bitand_is_zero(	mbedtls_mpi* a, mbedtls_mpi* b ) {
 
 mbedtls_mpi_uint mpi_get_uint(const mbedtls_mpi *X) {
 	
-	if (X->n == 1) {
+	if (X->n == 1 && X->s > 0) {
 		return X->p[0];
 	}