--- a/taskcluster/taskgraph/transforms/nightly_l10n_signing.py
+++ b/taskcluster/taskgraph/transforms/nightly_l10n_signing.py
@@ -26,34 +26,34 @@ def make_signing_description(config, job
             job_specs = [
                 {
                     'artifacts': ['public/build/{locale}/target.apk'],
                     'format': 'jar',
                 },
             ]
         elif 'macosx' in dep_platform:
             job_specs = [
-                 {
+                {
                     'artifacts': ['public/build/{locale}/target.dmg'],
                     'format': 'macapp',
-                 }, {
-                    'artifacts': ['public/build/{locale}/target.complete.mar'],
-                    'format': 'mar',
-                 }
+                }
             ]
-        else:
+        elif 'linux' in dep_platform:
             job_specs = [
                 {
                     'artifacts': ['public/build/{locale}/target.tar.bz2'],
                     'format': 'gpg',
                 }, {
                     'artifacts': ['public/build/{locale}/target.complete.mar'],
                     'format': 'mar',
                 }
             ]
+        else:
+            raise Exception("Platform not implemented for signing")
+
         upstream_artifacts = []
         for spec in job_specs:
             fmt = spec['format']
             upstream_artifacts.append({
                 "taskId": {"task-reference": "<unsigned-repack>"},
                 "taskType": "l10n",
                 # Set paths based on artifacts in the specs (above) one per
                 # locale present in the chunk this is signing stuff for.