diff --git a/unit_test/run_s3sdk.sh b/unit_test/run_s3sdk.sh
index 4589bcb5f6..18073e4570 100755
--- a/unit_test/run_s3sdk.sh
+++ b/unit_test/run_s3sdk.sh
@@ -14,7 +14,7 @@ isolate "testdir_uts3sdk"
 # Create an isolation path for S3; build on the isolation directory
 S3ISODIR="$ISODIR"
 S3ISOPATH="/${S3TESTSUBTREE}"
-S3ISOPATH="${S3ISOPATH}/$S3ISODIR"
+S3ISOPATH="/${S3ISOPATH}/$S3ISODIR"
 
 test_cleanup() {
 ${CMD} ${execdir}/../nczarr_test/s3util -u "${URL}" -k "${S3ISOPATH}" clear