diff --git a/beetsplug/thumbnails.py b/beetsplug/thumbnails.py
index 6fd8c8cbe..b5aa05b17 100644
--- a/beetsplug/thumbnails.py
+++ b/beetsplug/thumbnails.py
@@ -234,8 +234,13 @@ class GioURI(URIGetter):
         self.available = bool(self.libgio)
         if self.available:
             self.libgio.g_type_init()  # for glib < 2.36
+
+            self.libgio.g_file_get_uri.argtypes = [ctypes.c_char_p]
             self.libgio.g_file_new_for_path.restype = ctypes.c_void_p
+
             self.libgio.g_file_get_uri.argtypes = [ctypes.c_void_p]
+            self.libgio.g_file_get_uri.restype = ctypes.c_char_p
+
             self.libgio.g_object_unref.argtypes = [ctypes.c_void_p]
 
     def get_library(self):