diff --git a/src/dsp/rescaler.c b/src/dsp/rescaler.c
index b940e6dc..6841142c 100644
--- a/src/dsp/rescaler.c
+++ b/src/dsp/rescaler.c
@@ -173,10 +173,10 @@ void WebPRescalerExportRow(WebPRescaler* const wrk) {
       WebPRescalerExportRowExpand(wrk);
     } else if (wrk->fxy_scale) {
       WebPRescalerExportRowShrink(wrk);
-    } else {  // very special case for src = dst = 1x1
+    } else {  // special case
       int i;
+      assert(wrk->src_height == wrk->dst_height && wrk->x_add == 1);
       assert(wrk->src_width == 1 && wrk->dst_width <= 2);
-      assert(wrk->src_height == 1 && wrk->dst_height == 1);
       for (i = 0; i < wrk->num_channels * wrk->dst_width; ++i) {
         wrk->dst[i] = wrk->irow[i];
         wrk->irow[i] = 0;
diff --git a/src/utils/rescaler.c b/src/utils/rescaler.c
index 00c9300b..d2278a52 100644
--- a/src/utils/rescaler.c
+++ b/src/utils/rescaler.c
@@ -48,11 +48,15 @@ void WebPRescalerInit(WebPRescaler* const wrk, int src_width, int src_height,
   wrk->y_sub = wrk->y_expand ? y_sub - 1 : y_sub;
   wrk->y_accum = wrk->y_expand ? wrk->y_sub : wrk->y_add;
   if (!wrk->y_expand) {
-    // this is WEBP_RESCALER_FRAC(dst_height, x_add * y_add) without the cast.
+    // This is WEBP_RESCALER_FRAC(dst_height, x_add * y_add) without the cast.
+    // Its value is <= WEBP_RESCALER_ONE, because dst_height <= wrk->y_add, and
+    // wrk->x_add >= 1;
     const uint64_t ratio =
         (uint64_t)dst_height * WEBP_RESCALER_ONE / (wrk->x_add * wrk->y_add);
     if (ratio != (uint32_t)ratio) {
-      // We can't represent the ratio with the current fixed-point precision.
+      // When ratio == WEBP_RESCALER_ONE, we can't represent the ratio with the
+      // current fixed-point precision. This happens when src_height ==
+      // wrk->y_add (which == src_height), and wrk->x_add == 1.
       // => We special-case fxy_scale = 0, in WebPRescalerExportRow().
       wrk->fxy_scale = 0;
     } else {