diff --git a/openssl-sys/src/handwritten/asn1.rs b/openssl-sys/src/handwritten/asn1.rs
index 844f9102a9..21d1c331c2 100644
--- a/openssl-sys/src/handwritten/asn1.rs
+++ b/openssl-sys/src/handwritten/asn1.rs
@@ -58,3 +58,9 @@ const_ptr_api! {
         pub fn ASN1_STRING_to_UTF8(out: *mut *mut c_uchar, s: #[const_ptr_if(any(ossl110, libressl280))] ASN1_STRING) -> c_int;
     }
 }
+
+const_ptr_api! {
+    extern "C" {
+        pub fn i2d_ASN1_INTEGER(b: *mut BIO, x: #[const_ptr_if(ossl300)] ASN1_INTEGER) -> c_int;
+    }
+}