diff --git a/library/ssl_tls13_keys.c b/library/ssl_tls13_keys.c
index 54742f3..5a6204e 100644
--- a/library/ssl_tls13_keys.c
+++ b/library/ssl_tls13_keys.c
@@ -75,6 +75,8 @@
  *         the HkdfLabel structure on success.
  */
 
+static const char tls1_3_label_prefix[6] = "tls13 ";
+
 #define SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN( label_len, context_len ) \
     (   2                  /* expansion length           */ \
       + 1                  /* label length               */ \
@@ -84,6 +86,7 @@
 
 #define SSL_TLS1_3_KEY_SCHEDULE_MAX_HKDF_LABEL_LEN                      \
     SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN(                             \
+                     sizeof(tls1_3_label_prefix) +                      \
                      MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN,     \
                      MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_CONTEXT_LEN )
 
@@ -93,8 +96,8 @@
                             const unsigned char *ctx, size_t clen,
                             unsigned char *dst, size_t *dlen )
 {
-    const char label_prefix[6] = "tls13 ";
-    size_t total_label_len = sizeof( label_prefix ) + llen;
+    size_t total_label_len =
+        sizeof(tls1_3_label_prefix) + llen;
     size_t total_hkdf_lbl_len =
         SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN( total_label_len, clen );
 
@@ -106,8 +109,8 @@
 
     /* Add label incl. prefix */
     *p++ = (unsigned char)( total_label_len & 0xFF );
-    memcpy( p, label_prefix, sizeof(label_prefix) );
-    p += sizeof(label_prefix);
+    memcpy( p, tls1_3_label_prefix, sizeof(tls1_3_label_prefix) );
+    p += sizeof(tls1_3_label_prefix);
     memcpy( p, label, llen );
     p += llen;