diff --git a/source/planar_functions.cc b/source/planar_functions.cc
index 3b703920..89e58ff6 100644
--- a/source/planar_functions.cc
+++ b/source/planar_functions.cc
@@ -43,8 +43,11 @@ void CopyPlane(const uint8_t* src_y,
     dst_y = dst_y + (height - 1) * dst_stride_y;
     dst_stride_y = -dst_stride_y;
   }
-  // Coalesce rows.
-  if (src_stride_y == width && dst_stride_y == width) {
+  // Coalesce rows. Skip if width*height would overflow int — the row kernels
+  // (CopyRow_ERMS / CopyRow_C) sign-extend a negative width to a huge size_t
+  // and copy unbounded. Falling through copies the same bytes row-by-row.
+  if (src_stride_y == width && dst_stride_y == width &&
+      (int64_t)width * (int64_t)height <= 2147483647LL) {
     width *= height;
     height = 1;
     src_stride_y = dst_stride_y = 0;