diff --git a/glib/gtimer.c b/glib/gtimer.c
index a51b359ca..38262ff9e 100644
--- a/glib/gtimer.c
+++ b/glib/gtimer.c
@@ -491,7 +491,14 @@ g_time_val_from_iso8601 (const gchar *iso_date,
  * Use g_date_time_format() or g_strdup_printf() if a different
  * variation of ISO 8601 format is required.
  *
- * Returns: a newly allocated string containing an ISO 8601 date
+ * If @time_ represents a date which is too large to fit into a `struct tm`,
+ * %NULL will be returned. This is platform dependent, but it is safe to assume
+ * years up to 3000 are supported. The return value of g_time_val_to_iso8601()
+ * has been nullable since GLib 2.54; before then, GLib would crash under the
+ * same conditions.
+ *
+ * Returns: (nullable): a newly allocated string containing an ISO 8601 date,
+ *    or %NULL if @time_ was too large
  *
  * Since: 2.12
  */
@@ -518,6 +525,10 @@ g_time_val_to_iso8601 (GTimeVal *time_)
 #endif
 #endif
 
+  /* If the gmtime() call has failed, time_->tv_sec is too big. */
+  if (tm == NULL)
+    return NULL;
+
   if (time_->tv_usec != 0)
     {
       /* ISO 8601 date and time format, with fractionary seconds: