diff --git a/src/attribution/onCLS.ts b/src/attribution/onCLS.ts
index 5fca98c..b10c5a8 100644
--- a/src/attribution/onCLS.ts
+++ b/src/attribution/onCLS.ts
@@ -27,13 +27,15 @@ const getLargestLayoutShiftSource = (sources: LayoutShiftAttribution[]) => {
   return sources.find((s) => s.node && s.node.nodeType === 1) || sources[0];
 };
 
-const attributeCLS = (metric: CLSMetric): void => {
+const attributeCLS = (metric: CLSMetric): CLSMetricWithAttribution => {
+  const metricWithAttribution = metric as CLSMetricWithAttribution;
+
   if (metric.entries.length) {
     const largestEntry = getLargestLayoutShiftEntry(metric.entries);
     if (largestEntry && largestEntry.sources && largestEntry.sources.length) {
       const largestSource = getLargestLayoutShiftSource(largestEntry.sources);
       if (largestSource) {
-        (metric as CLSMetricWithAttribution).attribution = {
+        metricWithAttribution.attribution = {
           largestShiftTarget: getSelector(largestSource.node),
           largestShiftTime: largestEntry.startTime,
           largestShiftValue: largestEntry.value,
@@ -41,12 +43,13 @@ const attributeCLS = (metric: CLSMetric): void => {
           largestShiftEntry: largestEntry,
           loadState: getLoadState(largestEntry.startTime),
         };
-        return;
+        return metricWithAttribution;
       }
     }
   }
   // Set an empty object if no other attribution has been set.
-  (metric as CLSMetricWithAttribution).attribution = {};
+  metricWithAttribution.attribution = {};
+  return metricWithAttribution;
 };
 
 /**
@@ -75,7 +78,7 @@ export const onCLS = (
   opts?: ReportOpts,
 ) => {
   unattributedOnCLS((metric: CLSMetric) => {
-    attributeCLS(metric);
-    onReport(metric as CLSMetricWithAttribution);
+    const metricWithAttribution = attributeCLS(metric);
+    onReport(metricWithAttribution);
   }, opts);
 };
diff --git a/src/attribution/onFCP.ts b/src/attribution/onFCP.ts
index 649f96e..16df0ec 100644
--- a/src/attribution/onFCP.ts
+++ b/src/attribution/onFCP.ts
@@ -21,34 +21,38 @@ import {isInvalidTimestamp} from '../lib/isInvalidTimestamp.js';
 import {onFCP as unattributedOnFCP} from '../onFCP.js';
 import {FCPMetric, FCPMetricWithAttribution, ReportOpts} from '../types.js';
 
-const attributeFCP = (metric: FCPMetric): void => {
+const attributeFCP = (metric: FCPMetric): FCPMetricWithAttribution => {
+  const metricWithAttribution = metric as FCPMetricWithAttribution;
+
   if (metric.entries.length) {
     const navigationEntry = getNavigationEntry();
     const fcpEntry = metric.entries[metric.entries.length - 1];
 
     if (navigationEntry) {
       const responseStart = navigationEntry.responseStart;
-      if (isInvalidTimestamp(responseStart)) return;
+      // TODO(bckenny): this is wrong.
+      if (isInvalidTimestamp(responseStart)) return metricWithAttribution;
 
       const activationStart = navigationEntry.activationStart || 0;
       const ttfb = Math.max(0, responseStart - activationStart);
 
-      (metric as FCPMetricWithAttribution).attribution = {
+      metricWithAttribution.attribution = {
         timeToFirstByte: ttfb,
         firstByteToFCP: metric.value - ttfb,
         loadState: getLoadState(metric.entries[0].startTime),
         navigationEntry,
         fcpEntry,
       };
-      return;
+      return metricWithAttribution;
     }
   }
   // Set an empty object if no other attribution has been set.
-  (metric as FCPMetricWithAttribution).attribution = {
+  metricWithAttribution.attribution = {
     timeToFirstByte: 0,
     firstByteToFCP: metric.value,
     loadState: getLoadState(getBFCacheRestoreTime()),
   };
+  return metricWithAttribution;
 };
 
 /**
@@ -62,7 +66,7 @@ export const onFCP = (
   opts?: ReportOpts,
 ) => {
   unattributedOnFCP((metric: FCPMetric) => {
-    attributeFCP(metric);
-    onReport(metric as FCPMetricWithAttribution);
+    const metricWithAttribution = attributeFCP(metric);
+    onReport(metricWithAttribution);
   }, opts);
 };
diff --git a/src/attribution/onFID.ts b/src/attribution/onFID.ts
index a75a257..7a5143e 100644
--- a/src/attribution/onFID.ts
+++ b/src/attribution/onFID.ts
@@ -19,15 +19,18 @@ import {getSelector} from '../lib/getSelector.js';
 import {onFID as unattributedOnFID} from '../onFID.js';
 import {FIDMetric, FIDMetricWithAttribution, ReportOpts} from '../types.js';
 
-const attributeFID = (metric: FIDMetric): void => {
+const attributeFID = (metric: FIDMetric): FIDMetricWithAttribution => {
+  const metricWithAttribution = metric as FIDMetricWithAttribution;
+
   const fidEntry = metric.entries[0];
-  (metric as FIDMetricWithAttribution).attribution = {
+  metricWithAttribution.attribution = {
     eventTarget: getSelector(fidEntry.target),
     eventType: fidEntry.name,
     eventTime: fidEntry.startTime,
     eventEntry: fidEntry,
     loadState: getLoadState(fidEntry.startTime),
   };
+  return metricWithAttribution;
 };
 
 /**
@@ -44,7 +47,7 @@ export const onFID = (
   opts?: ReportOpts,
 ) => {
   unattributedOnFID((metric: FIDMetric) => {
-    attributeFID(metric);
-    onReport(metric as FIDMetricWithAttribution);
+    const metricWithAttribution = attributeFID(metric);
+    onReport(metricWithAttribution);
   }, opts);
 };
diff --git a/src/attribution/onINP.ts b/src/attribution/onINP.ts
index 378ad2c..e198330 100644
--- a/src/attribution/onINP.ts
+++ b/src/attribution/onINP.ts
@@ -186,7 +186,7 @@ const getIntersectingLoAFs = (
   return intersectingLoAFs;
 };
 
-const attributeINP = (metric: INPMetric): void => {
+const attributeINP = (metric: INPMetric): INPMetricWithAttribution => {
   const firstEntry = metric.entries[0];
   const renderTime = entryToRenderTimeMap.get(firstEntry)!;
   const group = pendingEntriesGroupMap.get(renderTime)!;
@@ -220,7 +220,8 @@ const attributeINP = (metric: INPMetric): void => {
 
   const nextPaintTime = Math.max.apply(Math, nextPaintTimeCandidates);
 
-  (metric as INPMetricWithAttribution).attribution = {
+  const metricWithAttribution = metric as INPMetricWithAttribution;
+  metricWithAttribution.attribution = {
     interactionTarget: getSelector(
       firstEntryWithTarget && firstEntryWithTarget.target,
     ),
@@ -234,6 +235,7 @@ const attributeINP = (metric: INPMetric): void => {
     presentationDelay: Math.max(nextPaintTime - processingEnd, 0),
     loadState: getLoadState(firstEntry.startTime),
   };
+  return metricWithAttribution;
 };
 
 /**
@@ -278,8 +280,8 @@ export const onINP = (
     // running in Chrome (EventTimingKeypressAndCompositionInteractionId)
     // 123+ that if rolled out fully would make this no longer necessary.
     whenIdle(() => {
-      attributeINP(metric);
-      onReport(metric as INPMetricWithAttribution);
+      const metricWithAttribution = attributeINP(metric);
+      onReport(metricWithAttribution);
     });
   }, opts);
 };
diff --git a/src/attribution/onLCP.ts b/src/attribution/onLCP.ts
index aeba48f..3e5b21b 100644
--- a/src/attribution/onLCP.ts
+++ b/src/attribution/onLCP.ts
@@ -25,13 +25,16 @@ import {
   ReportOpts,
 } from '../types.js';
 
-const attributeLCP = (metric: LCPMetric) => {
+const attributeLCP = (metric: LCPMetric): LCPMetricWithAttribution => {
+  const metricWithAttribution = metric as LCPMetricWithAttribution;
+
   if (metric.entries.length) {
     const navigationEntry = getNavigationEntry();
 
     if (navigationEntry) {
       const responseStart = navigationEntry.responseStart;
-      if (isInvalidTimestamp(responseStart)) return;
+      // TODO(bckenny): this is wrong.
+      if (isInvalidTimestamp(responseStart)) return metricWithAttribution;
 
       const activationStart = navigationEntry.activationStart || 0;
       const lcpEntry = metric.entries[metric.entries.length - 1];
@@ -78,17 +81,18 @@ const attributeLCP = (metric: LCPMetric) => {
         attribution.lcpResourceEntry = lcpResourceEntry;
       }
 
-      (metric as LCPMetricWithAttribution).attribution = attribution;
-      return;
+      metricWithAttribution.attribution = attribution;
+      return metricWithAttribution;
     }
   }
   // Set an empty object if no other attribution has been set.
-  (metric as LCPMetricWithAttribution).attribution = {
+  metricWithAttribution.attribution = {
     timeToFirstByte: 0,
     resourceLoadDelay: 0,
     resourceLoadDuration: 0,
     elementRenderDelay: metric.value,
   };
+  return metricWithAttribution;
 };
 
 /**
@@ -107,7 +111,7 @@ export const onLCP = (
   opts?: ReportOpts,
 ) => {
   unattributedOnLCP((metric: LCPMetric) => {
-    attributeLCP(metric);
-    onReport(metric as LCPMetricWithAttribution);
+    const metricWithAttribution = attributeLCP(metric);
+    onReport(metricWithAttribution);
   }, opts);
 };
diff --git a/src/attribution/onTTFB.ts b/src/attribution/onTTFB.ts
index 3f267c0..51c92a4 100644
--- a/src/attribution/onTTFB.ts
+++ b/src/attribution/onTTFB.ts
@@ -17,7 +17,9 @@
 import {onTTFB as unattributedOnTTFB} from '../onTTFB.js';
 import {TTFBMetric, TTFBMetricWithAttribution, ReportOpts} from '../types.js';
 
-const attributeTTFB = (metric: TTFBMetric): void => {
+const attributeTTFB = (metric: TTFBMetric): TTFBMetricWithAttribution => {
+  const metricWithAttribution = metric as TTFBMetricWithAttribution;
+
   if (metric.entries.length) {
     const navigationEntry = metric.entries[0];
     const activationStart = navigationEntry.activationStart || 0;
@@ -43,7 +45,7 @@ const attributeTTFB = (metric: TTFBMetric): void => {
       0,
     );
 
-    (metric as TTFBMetricWithAttribution).attribution = {
+    metricWithAttribution.attribution = {
       waitingDuration: waitEnd,
       cacheDuration: dnsStart - waitEnd,
       // dnsEnd usually equals connectStart but use connectStart over dnsEnd
@@ -57,16 +59,17 @@ const attributeTTFB = (metric: TTFBMetric): void => {
       requestDuration: metric.value - connectEnd,
       navigationEntry: navigationEntry,
     };
-    return;
+    return metricWithAttribution;
   }
   // Set an empty object if no other attribution has been set.
-  (metric as TTFBMetricWithAttribution).attribution = {
+  metricWithAttribution.attribution = {
     waitingDuration: 0,
     cacheDuration: 0,
     dnsDuration: 0,
     connectionDuration: 0,
     requestDuration: 0,
   };
+  return metricWithAttribution;
 };
 
 /**
@@ -89,7 +92,7 @@ export const onTTFB = (
   opts?: ReportOpts,
 ) => {
   unattributedOnTTFB((metric: TTFBMetric) => {
-    attributeTTFB(metric);
-    onReport(metric as TTFBMetricWithAttribution);
+    const metricWithAttribution = attributeTTFB(metric);
+    onReport(metricWithAttribution);
   }, opts);
 };