diff --git a/builds/libraries/vendor/libffi b/builds/libraries/vendor/libffi
index f189fa0..bfe7e38 100755
--- a/builds/libraries/vendor/libffi
+++ b/builds/libraries/vendor/libffi
@@ -4,7 +4,7 @@
 OUT_PREFIX=$1
 
 # Skip the build for heroku-16.
-if [[ S3_PREFIX == "heroku-16" ]]; then
+if [[ $S3_PREFIX == "heroku-16" ]]; then
     exit 0
 fi
 
diff --git a/builds/libraries/vendor/libmemcache b/builds/libraries/vendor/libmemcache
index a5b716f..05c3131 100755
--- a/builds/libraries/vendor/libmemcache
+++ b/builds/libraries/vendor/libmemcache
@@ -4,7 +4,7 @@
 OUT_PREFIX=$1
 
 # Skip the build for heroku-16.
-if [[ S3_PREFIX == "heroku-16" ]]; then
+if [[ $S3_PREFIX == "heroku-16" ]]; then
     exit 0
 fi