diff --git a/codecs/optipng/build.sh b/codecs/optipng/build.sh
index 5f4c3927..3a523b59 100755
--- a/codecs/optipng/build.sh
+++ b/codecs/optipng/build.sh
@@ -27,6 +27,7 @@ echo "============================================="
   emcc \
     --bind \
     ${OPTIMIZE} \
+    --closure 1 \
     -s ALLOW_MEMORY_GROWTH=1 -s MODULARIZE=1 -s 'EXPORT_NAME="optipng"' \
     -o "optipng.js" \
     --std=c++11 \