diff --git a/gio/gcontenttype.c b/gio/gcontenttype.c
index a9bdb4b57..dc48411e7 100644
--- a/gio/gcontenttype.c
+++ b/gio/gcontenttype.c
@@ -330,6 +330,10 @@ g_content_type_guess (const gchar  *filename,
   if (result_uncertain)
     *result_uncertain = FALSE;
 
+  /* our test suite and potentially other code used -1 in the past, which is
+   * not documented and not allowed; guard against that */
+  g_return_val_if_fail (data_size != (gsize) -1, g_strdup (XDG_MIME_TYPE_UNKNOWN));
+
   if (filename)
     {
       basename = g_path_get_basename (filename);
@@ -906,6 +910,10 @@ g_content_type_guess (const gchar  *filename,
   if (result_uncertain)
     *result_uncertain = FALSE;
 
+  /* our test suite and potentially other code used -1 in the past, which is
+   * not documented and not allowed; guard against that */
+  g_return_val_if_fail (data_size != (gsize) -1, g_strdup (XDG_MIME_TYPE_UNKNOWN));
+
   G_LOCK (gio_xdgmime);
 
   if (filename)
diff --git a/gio/tests/contenttype.c b/gio/tests/contenttype.c
index 71888944d..9e0f9af5d 100644
--- a/gio/tests/contenttype.c
+++ b/gio/tests/contenttype.c
@@ -55,14 +55,14 @@ test_guess (void)
   g_free (res);
   g_free (expected);
 
-  res = g_content_type_guess ("test.pot", (guchar *)"ABC abc", -1, &uncertain);
+  res = g_content_type_guess ("test.pot", (guchar *)"ABC abc", 7, &uncertain);
   expected = g_content_type_from_mime_type ("application/vnd.ms-powerpoint");
   g_assert (g_content_type_equals (expected, res));
   g_assert (uncertain);
   g_free (res);
   g_free (expected);
 
-  res = g_content_type_guess ("test.otf", (guchar *)"OTTO", -1, &uncertain);
+  res = g_content_type_guess ("test.otf", (guchar *)"OTTO", 4, &uncertain);
   expected = g_content_type_from_mime_type ("application/x-font-otf");
   g_assert (g_content_type_equals (expected, res));
   g_assert (!uncertain);