diff --git a/src/hardware/hw_main.c b/src/hardware/hw_main.c
index 9aec64fad..761190eb1 100644
--- a/src/hardware/hw_main.c
+++ b/src/hardware/hw_main.c
@@ -1667,7 +1667,7 @@ static void HWR_ProcessSeg(void) // Sort of like GLWall::Process in GZDoom
 
 				if (rover->master->flags & ML_TFERLINE)
 				{
-					size_t linenum = gl_curline->linedef-gl_backsector->lines[0];
+					size_t linenum = min(gl_curline->linedef-gl_backsector->lines[0], rover->master->frontsector->linecount);
 					newline = rover->master->frontsector->lines[0] + linenum;
 					texnum = R_GetTextureNum(sides[newline->sidenum[0]].midtexture);
 				}
@@ -1827,7 +1827,7 @@ static void HWR_ProcessSeg(void) // Sort of like GLWall::Process in GZDoom
 
 				if (rover->master->flags & ML_TFERLINE)
 				{
-					size_t linenum = gl_curline->linedef-gl_backsector->lines[0];
+					size_t linenum = min(gl_curline->linedef-gl_backsector->lines[0], rover->master->frontsector->linecount);
 					newline = rover->master->frontsector->lines[0] + linenum;
 					texnum = R_GetTextureNum(sides[newline->sidenum[0]].midtexture);
 				}
diff --git a/src/r_segs.c b/src/r_segs.c
index 7f190ea3f..d2f9db9a0 100644
--- a/src/r_segs.c
+++ b/src/r_segs.c
@@ -653,7 +653,7 @@ 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(curline->linedef-backsector->lines[0], pfloor->master->frontsector->linecount);
 		newline = pfloor->master->frontsector->lines[0] + linenum;
 		texnum = R_GetTextureNum(sides[newline->sidenum[0]].midtexture);
 		bmnum = R_GetTextureBrightmap(texnum);