diff --git a/src/nvim/ops.c b/src/nvim/ops.c
index b5408fab9a..b96e075f66 100644
--- a/src/nvim/ops.c
+++ b/src/nvim/ops.c
@@ -3749,7 +3749,7 @@ int do_join(size_t count,
   if (setmark) {
     // Set the '] mark.
     curwin->w_buffer->b_op_end.lnum = curwin->w_cursor.lnum;
-    curwin->w_buffer->b_op_end.col = (colnr_T)STRLEN(newp);
+    curwin->w_buffer->b_op_end.col = sumsize;
   }
 
   /* Only report the change in the first line here, del_lines() will report