diff --git a/kitty/core_text.m b/kitty/core_text.m
index ad3cd96e9..fd92d1389 100644
--- a/kitty/core_text.m
+++ b/kitty/core_text.m
@@ -757,6 +757,7 @@ render_sample_text(CTFace *self, PyObject *args) {
     CTFontGetAdvancesForGlyphs(font, kCTFontOrientationDefault, buffers.glyphs, local_advances, num_chars);
     CTFontGetBoundingRectsForGlyphs(font, kCTFontOrientationDefault, buffers.glyphs, buffers.boxes, num_chars);
     CGFloat x = 0, y = 0;
+    memset(PyByteArray_AS_STRING(pbuf), 0, PyBytes_GET_SIZE(pbuf));
     if (cell_width > canvas_width) goto end;
     for (size_t i = 0; i < num_chars; i++) {
         if (local_advances[i].width + x > canvas_width) {
diff --git a/kitty/freetype.c b/kitty/freetype.c
index 4e08bb76e..c34b74164 100644
--- a/kitty/freetype.c
+++ b/kitty/freetype.c
@@ -933,6 +933,7 @@ render_sample_text(Face *self, PyObject *args) {
     unsigned int cell_width, cell_height, baseline, underline_position, underline_thickness, strikethrough_position, strikethrough_thickness;
     cell_metrics((PyObject*)self, &cell_width, &cell_height, &baseline, &underline_position, &underline_thickness, &strikethrough_position, &strikethrough_thickness);
     pixel *canvas = (pixel*)PyBytes_AS_STRING(pbuf);
+    memset(canvas, 0, PyBytes_GET_SIZE(pbuf));
     if (cell_width > canvas_width) goto end;
 
     for (ssize_t n = 0; n < PyUnicode_GET_LENGTH(ptext); n++) {