diff --git a/kitty/line-buf.c b/kitty/line-buf.c
index 05fb72f39..8589f4db9 100644
--- a/kitty/line-buf.c
+++ b/kitty/line-buf.c
@@ -148,6 +148,9 @@ linebuf_init_line(LineBuf *self, index_type idx) {
 
 void
 linebuf_clear_lines(LineBuf *self, const Cursor *cursor, index_type start, index_type end) {
+#if BLANK_CHAR != 0
+#error This implementation is incorrect for BLANK_CHAR != 0
+#endif
     GPUCell *first_gpu_line = gpu_lineptr(self, start);
     GPUCell gc = cursor_as_gpu_cell(cursor);
     for (index_type i = 0; i < self->xnum; i++) memcpy(first_gpu_line + i, &gc, sizeof(gc));
diff --git a/kitty/line.c b/kitty/line.c
index 75f77c555..09f9cdaaa 100644
--- a/kitty/line.c
+++ b/kitty/line.c
@@ -542,6 +542,9 @@ void
 line_apply_cursor(Line *self, const Cursor *cursor, unsigned int at, unsigned int num, bool clear_char) {
     GPUCell gc = cursor_as_gpu_cell(cursor);
     if (clear_char) {
+#if BLANK_CHAR != 0
+#error This implementation is incorrect for BLANK_CHAR != 0
+#endif
         for (index_type i = at; i < self->xnum && i < at + num; i++) {
             memset(self->cpu_cells + i, 0, sizeof(self->cpu_cells[0]));
             memcpy(self->gpu_cells + i, &gc, sizeof(gc));