diff --git a/crypto/err/err.c b/crypto/err/err.c
index b78ee6b3..76d101ab 100644
--- a/crypto/err/err.c
+++ b/crypto/err/err.c
@@ -271,6 +271,11 @@ void ERR_clear_error(void) {
   state->top = state->bottom = 0;
 }
 
+int ERR_get_next_error_library() {
+  err_fns_check();
+  return ERRFN(get_next_library)();
+}
+
 void ERR_clear_system_error(void) {
   errno = 0;
 }
diff --git a/crypto/err/err.h b/crypto/err/err.h
index 411bc9be..891fb3e7 100644
--- a/crypto/err/err.h
+++ b/crypto/err/err.h
@@ -254,6 +254,14 @@ void ERR_print_errors_cb(ERR_print_errors_callback_t callback, void *ctx);
 void ERR_clear_error(void);
 
 
+/* Custom errors. */
+
+/* ERR_get_next_error_library returns a value suitable for passing as the
+ * |library| argument to |ERR_put_error|. This is intended for code that wishes
+ * to push its own, non-standard errors to the error queue. */
+int ERR_get_next_error_library();
+
+
 /* Private functions. */
 
 /* ERR_clear_system_error clears the system's error value (i.e. errno). */
@@ -467,6 +475,11 @@ struct ERR_FNS_st {
   /* get_state returns the ERR_STATE for the current thread. This function
    * never returns NULL. */
   ERR_STATE *(*get_state)(void);
+
+  /* get_next_library returns a unique value suitable for passing as the
+   * |library| to error calls. It will be distinct from all built-in library
+   * values. */
+  int (*get_next_library)(void);
 };
 
 /* OPENSSL_DECLARE_ERROR_REASON is used by util/make_errors.h (which generates
diff --git a/crypto/err/err_impl.c b/crypto/err/err_impl.c
index ff2509d4..6544c7b5 100644
--- a/crypto/err/err_impl.c
+++ b/crypto/err/err_impl.c
@@ -125,6 +125,9 @@ static LHASH_OF(ERR_STATE) *state_hash = NULL;
  * function / reason. */
 static LHASH_OF(ERR_STRING_DATA) *error_hash = NULL;
 
+/* global_next_library contains the next custom library value to return. */
+static int global_next_library = ERR_NUM_LIBS;
+
 /* err_string_data_hash is an lhash hash function for ERR_STRING_DATA. */
 static uint32_t err_string_data_hash(const ERR_STRING_DATA *a) {
   return OPENSSL_hash32(&a->error, sizeof(a->error));
@@ -271,10 +274,21 @@ static void err_shutdown(void) {
   CRYPTO_w_unlock(CRYPTO_LOCK_ERR);
 }
 
+static int err_get_next_library(void) {
+  int ret;
+
+  CRYPTO_w_lock(CRYPTO_LOCK_ERR);
+  ret = global_next_library++;
+  CRYPTO_w_unlock(CRYPTO_LOCK_ERR);
+
+  return ret;
+}
+
 const struct ERR_FNS_st openssl_err_default_impl = {
   err_shutdown,
   err_get_item,
   err_set_item,
   err_del_item,
   err_get_state,
+  err_get_next_library,
 };