diff --git a/extra/rust/build b/extra/rust/build
index 278dfa37..bae57a76 100755
--- a/extra/rust/build
+++ b/extra/rust/build
@@ -48,7 +48,7 @@ rust_version="${rust_version#rustc }"
 
 # If possible, use system rust to bootstrap.
 case "$rust_version" in
-    $2|$maj.$(($min - 1)).*)
+    "$2"|"$maj.$((min - 1))".*)
         cat >> config.toml <<EOF
 cargo          = "/usr/bin/cargo"
 rustc          = "/usr/bin/rustc"