diff --git a/include/openssl/base.h b/include/openssl/base.h
index 8be10d1..01a7e07 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -328,8 +328,11 @@
 // CRYPTO_THREADID is a dummy value.
 typedef int CRYPTO_THREADID;
 
+// An |ASN1_NULL| is an opaque type. asn1.h represents the ASN.1 NULL value as
+// an opaque, non-NULL |ASN1_NULL*| pointer.
+typedef struct asn1_null_st ASN1_NULL;
+
 typedef int ASN1_BOOLEAN;
-typedef int ASN1_NULL;
 typedef struct ASN1_ITEM_st ASN1_ITEM;
 typedef struct asn1_object_st ASN1_OBJECT;
 typedef struct asn1_pctx_st ASN1_PCTX;