diff --git a/src/build.sh b/src/build.sh
index bf91df4..6805088 100644
--- a/src/build.sh
+++ b/src/build.sh
@@ -2,7 +2,7 @@
 #! nix-shell -i bash
 #! nix-shell -p bash curl gcc zlib libffi icu77
 SUBVERSION="140.4.0"
-VERSION=$SUBVERSION+"esr"
+VERSION="${SUBVERSION}esr"
 DOWNLOAD="https://ftp.mozilla.org/pub/firefox/releases/$VERSION/source/firefox-$VERSION.source.tar.xz"
 BUILDPATH=$(realpath .)
 mkdir bin