diff --git a/wolfssl/wolfcrypt/asn.h b/wolfssl/wolfcrypt/asn.h
index 5eee741bd..860a7a1d3 100644
--- a/wolfssl/wolfcrypt/asn.h
+++ b/wolfssl/wolfcrypt/asn.h
@@ -66,12 +66,14 @@
     extern "C" {
 #endif
 
+#ifndef EXTERNAL_SERIAL_SIZE
+    #define EXTERNAL_SERIAL_SIZE 32
+#endif
+
 enum {
     ISSUER  = 0,
     SUBJECT = 1,
 
-    EXTERNAL_SERIAL_SIZE = 32,
-
     BEFORE  = 0,
     AFTER   = 1
 };