diff --git a/etc/ci/taskcluster/decision-task.py b/etc/ci/taskcluster/decision-task.py
index ddcd286cb1e..2f038b5493e 100644
--- a/etc/ci/taskcluster/decision-task.py
+++ b/etc/ci/taskcluster/decision-task.py
@@ -24,6 +24,7 @@ build_env = {
     "SCCACHE_IDLE_TIMEOUT": "1200",
     "CCACHE": "sccache",
     "RUSTC_WRAPPER": "sccache",
+    "SHELL": "/bin/dash",  # For SpiderMonkey’s build system
 }