diff --git a/common/src/util/ttft-histogram.ts b/common/src/util/ttft-histogram.ts
index f6c1820010..8c2f582f22 100644
--- a/common/src/util/ttft-histogram.ts
+++ b/common/src/util/ttft-histogram.ts
@@ -37,7 +37,8 @@ const LN_BASE = Math.log(TTFT_HISTOGRAM_BASE)
  * Sub-millisecond and zero samples land in bucket 0 rather than at -Infinity.
  */
 export function ttftBucketIndex(ttftMs: number): number {
-  const index = Math.floor(Math.log(Math.max(ttftMs, 1)) / LN_BASE)
+  const safeTtftMs = Number.isFinite(ttftMs) ? ttftMs : 0
+  const index = Math.floor(Math.log(Math.max(safeTtftMs, 1)) / LN_BASE)
   return Math.min(TTFT_HISTOGRAM_BUCKET_COUNT - 1, Math.max(0, index))
 }