diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 498b8b0..8f4a4f5 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -233,7 +233,7 @@
 int mbedtls_x509write_crt_set_key_usage( mbedtls_x509write_cert *ctx,
                                          unsigned int key_usage )
 {
-    unsigned char buf[5], ku[2];
+    unsigned char buf[5] = {0}, ku[2] = {0};
     unsigned char *c;
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     const unsigned int allowed_bits = MBEDTLS_X509_KU_DIGITAL_SIGNATURE |
@@ -272,7 +272,7 @@
 int mbedtls_x509write_crt_set_ns_cert_type( mbedtls_x509write_cert *ctx,
                                     unsigned char ns_cert_type )
 {
-    unsigned char buf[4];
+    unsigned char buf[4] = {0};
     unsigned char *c;
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;