diff --git a/src/p_floor.c b/src/p_floor.c
index a0899e88..8a38840c 100644
--- a/src/p_floor.c
+++ b/src/p_floor.c
@@ -926,6 +926,9 @@ int EV_DoDonut(line_t*  line)
       //jff 3/29/98 use true two-sidedness, not the flag
       if (comp[comp_model])
       {
+        // original code:   !s2->lines[i]->flags & ML_TWOSIDED
+        // equivalent to:   (!s2->lines[i]->flags) & ML_TWOSIDED , i.e. 0
+        // should be:       !(s2->lines[i]->flags & ML_TWOSIDED)
         if (((!s2->lines[i]->flags) & ML_TWOSIDED) ||
             (s2->lines[i]->backsector == s1))
           continue;