diff --git a/build.sh b/build.sh
index 88b6aa7..393302d 100755
--- a/build.sh
+++ b/build.sh
@@ -1,4 +1,6 @@
 #!/bin/bash
+# enable globbing properly
+shopt -s extglob
 
 # create the build dir if it doesn't already exist
 mkdir -p ./build
@@ -30,7 +32,7 @@ if [[ -e oxipng.tar.gz ]]; then
     mv ./oxipng* ./oxipngdir
     mv ./oxipngdir/oxipng ./oxipng
     chmod +x oxipng
-    oxipng -o max --strip all --alpha **/*.png
+    oxipng -o max --strip all --alpha $(find . -name "*.png" -type f)
 fi
 
 # remove all the gimp files (those don't do anything in the final build)