diff --git a/pl_mpeg.h b/pl_mpeg.h
index b42f53f..ab99610 100755
--- a/pl_mpeg.h
+++ b/pl_mpeg.h
@@ -3356,8 +3356,17 @@ void plm_video_process_macroblock(
 	unsigned int si = ((self->mb_row * block_size) + vp) * dw + (self->mb_col * block_size) + hp;
 	unsigned int di = (self->mb_row * dw + self->mb_col) * block_size;
 	
-	unsigned int max_address = (dw * (self->mb_height * block_size - block_size + 1) - block_size);
-	if (si > max_address || di > max_address) {
+	// The half-pel prediction reads s[si+1], s[si+dw] and s[si+dw+1], and the
+	// block loop spans block_size rows/cols, so the deepest source byte read is
+	// si + (block_size-1)*dw + (block_size-1) + (odd_v?dw:0) + (odd_h?1:0).
+	// Bound the actual deepest source AND destination offsets against the
+	// plane, not just the base indices si/di.
+	unsigned int plane_size = dw * (self->mb_height * block_size);
+	unsigned int max_di = di + (block_size - 1) * dw + (block_size - 1);
+	unsigned int max_si = si + (block_size - 1) * dw + (block_size - 1)
+		+ (odd_v ? dw : 0) + (odd_h ? 1 : 0);
+	if (si >= plane_size || di >= plane_size ||
+		max_si >= plane_size || max_di >= plane_size) {
 		return; // corrupt video
 	}