diff --git a/camkes-vm/build.py b/camkes-vm/build.py
index de8ebbe8..d7d18347 100644
--- a/camkes-vm/build.py
+++ b/camkes-vm/build.py
@@ -44,7 +44,7 @@ def run_build(manifest_dir: str, build: Build):
     ]
 
     if plat.has_simulation and plat.name != 'PC99':
-        script.append(sim_script(build.success))
+        script.append(sim_script(build.success, failure=build.error))
 
     return run_build_script(manifest_dir, build, script)
 
diff --git a/seL4-platforms/builds.py b/seL4-platforms/builds.py
index ca0df1a3..4f7aa2fa 100644
--- a/seL4-platforms/builds.py
+++ b/seL4-platforms/builds.py
@@ -553,11 +553,14 @@ def summarise_junit(file_path: str) -> Tuple[int, List[str]]:
                   "-q", junit_results, parsed_junit_results]
 
 
-def sim_script(success: str, timeout=1200):
+def sim_script(success: str, failure=None, timeout=1200):
+    """Return a script to run a simulation with timeout and expected success string.
+       Abort early on expected failure string."""
+    fail_str = "%s {exit 1}" % failure if failure else ""
     return [
         "expect", "-c",
-        'spawn ./simulate; set timeout %d; expect { "%s" {exit 0} timeout {exit 1} }' %
-        (timeout, success)
+        'spawn ./simulate; set timeout %d; expect "%s" {exit 0} %s timeout {exit 1}' %
+        (timeout, fail_str, success)
     ]
 
 
diff --git a/sel4test-sim/build.py b/sel4test-sim/build.py
index 9930f2da..2037bb65 100644
--- a/sel4test-sim/build.py
+++ b/sel4test-sim/build.py
@@ -18,7 +18,7 @@
 def run_simulation(manifest_dir: str, build: Build):
     """Run one simulation build and test."""
 
-    expect = '{ "%s" {exit 0} timeout {exit 1} }' % build.success
+    expect = '"%s" {exit 0} timeout {exit 1}' % build.success
 
     script = [
         ["../init-build.sh"] + build.settings_args(),