diff --git a/src/hardware/hw_main.c b/src/hardware/hw_main.c
index 88f92dbb..ba221646 100644
--- a/src/hardware/hw_main.c
+++ b/src/hardware/hw_main.c
@@ -1639,7 +1639,7 @@ void HWR_ProcessSeg(void) // Sort of like GLWall::Process in GZDoom
 
 				if (rover->master->flags & ML_TFERLINE)
 				{
-					size_t linenum = gr_curline->linedef-gr_backsector->lines[0];
+					size_t linenum = min((size_t)(gr_curline->linedef-gr_backsector->lines[0]), rover->master->frontsector->linecount);
 					newline = rover->master->frontsector->lines[0] + linenum;
 					texnum = R_GetTextureNum(sides[newline->sidenum[0]].midtexture);
 				}
@@ -1798,10 +1798,11 @@ void HWR_ProcessSeg(void) // Sort of like GLWall::Process in GZDoom
 
 				if (rover->master->flags & ML_TFERLINE)
 				{
-					size_t linenum = gr_curline->linedef-gr_backsector->lines[0];
+					size_t linenum = min((size_t)(gr_curline->linedef-gr_backsector->lines[0]), rover->master->frontsector->linecount);
 					newline = rover->master->frontsector->lines[0] + linenum;
 					texnum = R_GetTextureNum(sides[newline->sidenum[0]].midtexture);
 				}
+
 #ifdef ESLOPE //backsides
 				h  = *rover->t_slope ? P_GetZAt(*rover->t_slope, v1x, v1y) : *rover->topheight;
 				hS = *rover->t_slope ? P_GetZAt(*rover->t_slope, v2x, v2y) : *rover->topheight;
diff --git a/src/r_segs.c b/src/r_segs.c
index d3fa1c19..d3ed70c9 100644
--- a/src/r_segs.c
+++ b/src/r_segs.c
@@ -733,7 +733,7 @@ void R_RenderThickSideRange(drawseg_t *ds, INT32 x1, INT32 x2, ffloor_t *pfloor)
 
 	if (pfloor->master->flags & ML_TFERLINE)
 	{
-		size_t linenum = curline->linedef-backsector->lines[0];
+		size_t linenum = min((size_t)(curline->linedef-backsector->lines[0]), pfloor->master->frontsector->linecount);
 		newline = pfloor->master->frontsector->lines[0] + linenum;
 		texnum = R_GetTextureNum(sides[newline->sidenum[0]].midtexture);
 	}