diff --git a/src/utils/roll-spec-weights.ts b/src/utils/roll-spec-weights.ts
index a679d11..05bd205 100644
--- a/src/utils/roll-spec-weights.ts
+++ b/src/utils/roll-spec-weights.ts
@@ -109,18 +109,19 @@ export async function computeRefresh(
   const runs: Weights[] = [];
   const usedRunIds: number[] = [];
   for (const runId of runIds) {
-    let jobs: JobTimings[];
+    let run: Weights;
     try {
-      jobs = await fetchRunTimings(octokit, runId);
+      const jobs = await fetchRunTimings(octokit, runId);
+      if (!jobs.length) {
+        d(`run ${runId} has no timing artifacts (expired?) - ignoring`);
+        continue;
+      }
+      run = aggregateRun(jobs);
     } catch (e) {
       d(`run ${runId}: could not read its timing artifacts (${e.message}) - ignoring`);
       continue;
     }
-    if (!jobs.length) {
-      d(`run ${runId} has no timing artifacts (expired?) - ignoring`);
-      continue;
-    }
-    runs.push(aggregateRun(jobs));
+    runs.push(run);
     usedRunIds.push(runId);
   }
   if (!runs.length) {
diff --git a/src/utils/spec-weights.ts b/src/utils/spec-weights.ts
index f4758a9..ee0528f 100644
--- a/src/utils/spec-weights.ts
+++ b/src/utils/spec-weights.ts
@@ -258,7 +258,11 @@ export function timingsFromArtifactZip(zip: Buffer): SpecTimings[] {
     const data = zip.subarray(dataStart, dataStart + compressedSize);
     const raw = method === 0 ? data : method === 8 ? inflateRawSync(data) : null;
     if (!raw) throw new Error(`unsupported zip compression method ${method} for ${name}`);
-    found.push(JSON.parse(raw.toString('utf8')));
+    const timings = JSON.parse(raw.toString('utf8'));
+    if (!timings || typeof timings.files !== 'object' || timings.files === null) {
+      throw new Error(`${name} carries no per-file timings`);
+    }
+    found.push(timings);
   }
   return found;
 }
diff --git a/tests/fixtures/spec-weights/test_artifacts_truncated_1.zip b/tests/fixtures/spec-weights/test_artifacts_truncated_1.zip
new file mode 100644
index 0000000..e1b1051
Binary files /dev/null and b/tests/fixtures/spec-weights/test_artifacts_truncated_1.zip differ
diff --git a/tests/utils/spec-weights.spec.ts b/tests/utils/spec-weights.spec.ts
index 06d3026..e9586db 100644
--- a/tests/utils/spec-weights.spec.ts
+++ b/tests/utils/spec-weights.spec.ts
@@ -28,6 +28,11 @@ describe('spec-weights', () => {
       expect(timings[0].files['spec/api-browser-window-spec.ts']).toBeCloseTo(364.971, 3);
     });
 
+    it('rejects a timings file with no per-file data', () => {
+      const zip = readFileSync(join(fixtures, 'test_artifacts_truncated_1.zip'));
+      expect(() => timingsFromArtifactZip(zip)).toThrow(/no per-file timings/);
+    });
+
     it('rejects something that is not a zip', () => {
       expect(() => timingsFromArtifactZip(Buffer.from('nope'))).toThrow(/not a zip/);
     });