diff --git a/static/app/utils/replays/getFrameDetails.spec.tsx b/static/app/utils/replays/getFrameDetails.spec.tsx
index ec343d1cd950..ec68254e3306 100644
--- a/static/app/utils/replays/getFrameDetails.spec.tsx
+++ b/static/app/utils/replays/getFrameDetails.spec.tsx
@@ -20,4 +20,51 @@ describe('getFrameDetails', () => {
       expect(getFrameDetails(frame).colorGraphicsToken).toBe(color);
     });
   });
+
+  describe('defaultMapper (unknown frame types)', () => {
+    it('uses frame.message when it is a string', () => {
+      const frame: any = {
+        op: 'unknown.custom',
+        category: 'unknown.custom',
+        timestamp: new Date('2024-01-01T00:00:00Z').getTime() / 1000,
+        offsetMs: 0,
+        timestampMs: 0,
+        message: 'hello',
+        data: {type: 'foo'},
+      };
+      expect(getFrameDetails(frame).description).toBe('hello');
+    });
+
+    it('falls back to frame.data when frame.message is a non-string object', () => {
+      // frame.message could be a non-string at runtime (malformed frame data).
+      // ?? only guards null/undefined, so without a typeof check the whole frame
+      // object could become the description. We guard with typeof so the plain
+      // object falls through to frame.data instead.
+      const dataPayload = {type: 'foo', level: 'info', offsetMs: 0};
+      const frame: any = {
+        op: 'unknown.custom',
+        category: 'unknown.custom',
+        timestamp: new Date('2024-01-01T00:00:00Z').getTime() / 1000,
+        offsetMs: 0,
+        timestampMs: 0,
+        message: {nested: 'object'}, // non-string at runtime
+        data: dataPayload,
+      };
+      // Should fall back to frame.data, not use the non-string frame.message
+      expect(getFrameDetails(frame).description).toBe(dataPayload);
+    });
+
+    it('returns empty string when both message and data are absent', () => {
+      const frame: any = {
+        op: 'unknown.custom',
+        category: 'unknown.custom',
+        timestamp: new Date('2024-01-01T00:00:00Z').getTime() / 1000,
+        offsetMs: 0,
+        timestampMs: 0,
+        message: undefined,
+        data: undefined,
+      };
+      expect(getFrameDetails(frame).description).toBe('');
+    });
+  });
 });
diff --git a/static/app/utils/replays/getFrameDetails.tsx b/static/app/utils/replays/getFrameDetails.tsx
index 1145f272bc11..4109410c854c 100644
--- a/static/app/utils/replays/getFrameDetails.tsx
+++ b/static/app/utils/replays/getFrameDetails.tsx
@@ -468,7 +468,12 @@ const MAPPER_FOR_FRAME: Record<string, (frame: any) => Details> = {
 // JSX as components, and this one is called from event handlers.
 const defaultMapper = (frame: any): Details => ({
   colorGraphicsToken: 'neutral',
-  description: frame.message ?? frame.data ?? '',
+  // frame.message may be a non-string object at runtime (e.g. a malformed frame).
+  // The ?? operator only guards null/undefined, so we guard with typeof first.
+  // Passing a plain object is fine: BreadcrumbItem routes it to BreadcrumbStructuredData
+  // which renders it as an interactive inspector via StructuredEventData.
+  description:
+    (typeof frame.message === 'string' ? frame.message : undefined) ?? frame.data ?? '',
   tabKey: TabKey.BREADCRUMBS,
   title: toTitleCase(defaultTitle(frame)),
   icon: <IconTerminal size="xs" />,