diff --git a/box-drawing.c b/box-drawing.c
index 15b6e5c6..fb6f8d1b 100644
--- a/box-drawing.c
+++ b/box-drawing.c
@@ -1512,7 +1512,7 @@ draw_glyph(wchar_t wc, uint8_t *buf, int width, int height, int stride, int dpi)
 }
 
 struct fcft_glyph *
-box_drawing(struct terminal *term, wchar_t wc)
+box_drawing(const struct terminal *term, wchar_t wc)
 {
     int width = term->cell_width;
     int height = term->cell_height;
diff --git a/box-drawing.h b/box-drawing.h
index aecbf1b3..e4633c7e 100644
--- a/box-drawing.h
+++ b/box-drawing.h
@@ -3,4 +3,4 @@
 #include <fcft/fcft.h>
 
 struct terminal;
-struct fcft_glyph *box_drawing(struct terminal *term, wchar_t wc);
+struct fcft_glyph *box_drawing(const struct terminal *term, wchar_t wc);