diff --git a/src/fold.c b/src/fold.c
index 33c2699..8078e3a 100644
--- a/src/fold.c
+++ b/src/fold.c
@@ -3066,7 +3066,7 @@
 
 #define fold_end(fp) ((fp)->fd_top + (fp)->fd_len - 1)
 #define valid_fold(fp, gap) ((gap)->ga_len > 0 && (fp) < ((fold_T *)(gap)->ga_data + (gap)->ga_len))
-#define fold_index(fp, gap) ((size_t)(fp - ((fold_T *)(gap)->ga_data)))
+#define fold_index(fp, gap) ((size_t)((fp) - ((fold_T *)(gap)->ga_data)))
 
     void
 foldMoveRange(garray_T *gap, linenr_T line1, linenr_T line2, linenr_T dest)