diff --git a/Modules/_ctypes/callbacks.c b/Modules/_ctypes/callbacks.c
--- a/Modules/_ctypes/callbacks.c
+++ b/Modules/_ctypes/callbacks.c
@@ -345,6 +345,7 @@
     p->callable = NULL;
     p->setfunc = NULL;
     p->ffi_restype = NULL;
+    p->restype = NULL;
 
     for (i = 0; i < nArgs + 1; ++i)
         p->atypes[i] = NULL;