diff --git a/src/userfunc.c b/src/userfunc.c
index 8bfebec..e76e6d8 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -25,7 +25,7 @@
 
 /* From user function to hashitem and back. */
 #define UF2HIKEY(fp) ((fp)->uf_name)
-#define HIKEY2UF(p)  ((ufunc_T *)(p - offsetof(ufunc_T, uf_name)))
+#define HIKEY2UF(p)  ((ufunc_T *)((p) - offsetof(ufunc_T, uf_name)))
 #define HI2UF(hi)     HIKEY2UF((hi)->hi_key)
 
 #define FUNCARG(fp, j)	((char_u **)(fp->uf_args.ga_data))[j]