diff --git a/CHANGES.md b/CHANGES.md
index b531481..b45b6d3 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -4,6 +4,10 @@
 - Add `digest` function (#65, #66).
 - Restore compatibility with openssl < 1.1.0 (#73).
 - Improved compatibility with OCaml 5 (#79).
+- Fix `client_verify_callback` for `NO_NAKED_POINTERS` mode. A user-provided
+  verification function in C remains an out-of-heap pointer for 4.x for
+  compatibility, but is boxed for OCaml 5.x or 4.x when configured with
+  `--disable-naked-pointers`. (#83)
 
 0.5.10 (2021-02-01)
 ======
diff --git a/src/ssl_stubs.c b/src/ssl_stubs.c
index 2822d81..69beca1 100644
--- a/src/ssl_stubs.c
+++ b/src/ssl_stubs.c
@@ -59,6 +59,9 @@
 #endif
 
 static int client_verify_callback(int, X509_STORE_CTX *);
+#ifdef NO_NAKED_POINTERS
+static value vclient_verify_callback = Val_int(0);
+#endif
 static DH *load_dh_param(const char *dhfile);
 
 /*******************
@@ -561,7 +564,16 @@ CAMLprim value ocaml_ssl_digest(value vevp, value vcert)
 
 CAMLprim value ocaml_ssl_get_client_verify_callback_ptr(value unit)
 {
+#ifdef NO_NAKED_POINTERS
+  if (Is_long(vclient_verify_callback)) {
+    vclient_verify_callback = caml_alloc_shr(1, Abstract_tag);
+    *((int(**) (int, X509_STORE_CTX*))Data_abstract_val(vclient_verify_callback)) = client_verify_callback;
+    caml_register_generational_global_root(&vclient_verify_callback);
+  }
+  return vclient_verify_callback;
+#else
   return (value)client_verify_callback;
+#endif
 }
 
 static int client_verify_callback_verbose = 1;
@@ -610,7 +622,16 @@ CAMLprim value ocaml_ssl_ctx_set_verify(value context, value vmode, value vcallb
   }
 
   if (Is_block(vcallback))
+  {
+#ifdef NO_NAKED_POINTERS
+    vcallback = Field(vcallback, 0);
+    if (!Is_block(vcallback) || Tag_val(vcallback) != Abstract_tag || Wosize_val(vcallback) != 1)
+      caml_invalid_argument("callback");
+    callback = *((int(**) (int, X509_STORE_CTX*))Data_abstract_val(vcallback));
+#else
     callback = (int(*) (int, X509_STORE_CTX*))Field(vcallback, 0);
+#endif
+  }
 
   caml_enter_blocking_section();
   SSL_CTX_set_verify(ctx, mode, callback);