diff --git a/src/libostree/ostree-gpg-verify-result.c b/src/libostree/ostree-gpg-verify-result.c
index 73fbfeed..cd709b7c 100644
--- a/src/libostree/ostree-gpg-verify-result.c
+++ b/src/libostree/ostree-gpg-verify-result.c
@@ -562,6 +562,14 @@ ostree_gpg_verify_result_describe_variant (GVariant *variant,
   key_id = (len > 16) ? fingerprint + len - 16 : fingerprint;
 
   date_time_utc = g_date_time_new_from_unix_utc (timestamp);
+  if (date_time_utc == NULL)
+    {
+      g_string_append_printf (output_buffer,
+                              "Can't check signature: timestamp %" G_GINT64_FORMAT " is invalid\n",
+                              timestamp);
+      return;
+    }
+
   date_time_local = g_date_time_to_local (date_time_utc);
   formatted_date_time = g_date_time_format (date_time_local, "%c");
 
@@ -606,6 +614,14 @@ ostree_gpg_verify_result_describe_variant (GVariant *variant,
   if (exp_timestamp > 0)
     {
       date_time_utc = g_date_time_new_from_unix_utc (exp_timestamp);
+      if (date_time_utc == NULL)
+        {
+          g_string_append_printf (output_buffer,
+                                  "Signature expiry timestamp (%" G_GINT64_FORMAT ") is invalid\n",
+                                  exp_timestamp);
+          return;
+        }
+
       date_time_local = g_date_time_to_local (date_time_utc);
       formatted_date_time = g_date_time_format (date_time_local, "%c");
 
diff --git a/src/libostree/ostree-sysroot-upgrader.c b/src/libostree/ostree-sysroot-upgrader.c
index 4e7c8bf3..11d9706c 100644
--- a/src/libostree/ostree-sysroot-upgrader.c
+++ b/src/libostree/ostree-sysroot-upgrader.c
@@ -470,8 +470,15 @@ ostree_sysroot_upgrader_check_timestamps (OstreeRepo     *repo,
       g_autofree char *old_ts_str = NULL;
       g_autofree char *new_ts_str = NULL;
 
-      g_assert (old_ts);
-      g_assert (new_ts);
+      if (old_ts == NULL || new_ts == NULL)
+        {
+          g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                       "Upgrade target revision '%s' timestamp (%" G_GINT64_FORMAT ") or current revision '%s' timestamp (%" G_GINT64_FORMAT ") is invalid",
+                       to_rev, ostree_commit_get_timestamp (new_commit),
+                       from_rev, ostree_commit_get_timestamp (old_commit));
+          goto out;
+        }
+
       old_ts_str = g_date_time_format (old_ts, "%c");
       new_ts_str = g_date_time_format (new_ts, "%c");
       g_date_time_unref (old_ts);
diff --git a/src/ostree/ot-remote-cookie-util.c b/src/ostree/ot-remote-cookie-util.c
index a96038aa..e3ca9eac 100644
--- a/src/ostree/ot-remote-cookie-util.c
+++ b/src/ostree/ot-remote-cookie-util.c
@@ -298,14 +298,18 @@ ot_list_cookies_at (int dfd, const char *jar_path, GError **error)
   while (ot_parse_cookies_next (parser))
     {
       g_autoptr(GDateTime) expires = g_date_time_new_from_unix_utc (parser->expiration);
-      g_autofree char *expires_str = g_date_time_format (expires, "%Y-%m-%d %H:%M:%S +0000");
+      g_autofree char *expires_str = NULL;
+
+      if (expires != NULL)
+        expires_str = g_date_time_format (expires, "%Y-%m-%d %H:%M:%S +0000");
 
       g_print ("--\n");
       g_print ("Domain: %s\n", parser->domain);
       g_print ("Path: %s\n", parser->path);
       g_print ("Name: %s\n", parser->name);
       g_print ("Secure: %s\n", parser->secure);
-      g_print ("Expires: %s\n", expires_str);
+      if (expires_str != NULL)
+        g_print ("Expires: %s\n", expires_str);
       g_print ("Value: %s\n", parser->value);
     }
 #else