diff --git a/core/src/exchanges/gemini-titan/fetcher.ts b/core/src/exchanges/gemini-titan/fetcher.ts
index c2014174..ae8b6333 100644
--- a/core/src/exchanges/gemini-titan/fetcher.ts
+++ b/core/src/exchanges/gemini-titan/fetcher.ts
@@ -106,11 +106,18 @@ export class GeminiFetcher implements IExchangeFetcher<GeminiRawEvent, GeminiRaw
     }
 
     async fetchRawOrderBook(instrumentSymbol: string): Promise<GeminiRawOrderBook | undefined> {
-        const eventTicker = this.getEventTickerForSymbol(instrumentSymbol);
+        let eventTicker = this.getEventTickerForSymbol(instrumentSymbol);
+        if (!eventTicker) {
+            // The symbol index is populated as a side effect of fetchRawEvents/
+            // fetchRawMarkets. A freshly constructed fetcher (e.g. a new instance
+            // created per server request for a credentialed client) has an empty
+            // index, so lazily build it once before giving up.
+            await this.fetchRawEvents({});
+            eventTicker = this.getEventTickerForSymbol(instrumentSymbol);
+        }
         if (!eventTicker) {
             throw new Error(
-                `Cannot fetch order book: no event ticker found for ${instrumentSymbol}. ` +
-                'Call fetchMarkets first to build the symbol index.',
+                `Cannot fetch order book: no event ticker found for ${instrumentSymbol}.`,
             );
         }
 
diff --git a/core/test/exchanges/gemini-titan-fetcher.test.ts b/core/test/exchanges/gemini-titan-fetcher.test.ts
index 931e1834..a81ccaeb 100644
--- a/core/test/exchanges/gemini-titan-fetcher.test.ts
+++ b/core/test/exchanges/gemini-titan-fetcher.test.ts
@@ -82,3 +82,67 @@ describe('GeminiFetcher authenticated orders', () => {
         await expect(fetcher.cancelRawOrder(123)).resolves.toBe(rawOrder);
     });
 });
+
+describe('GeminiFetcher order book symbol index', () => {
+    const eventsResponse = {
+        data: [
+            {
+                ticker: 'EVT-1',
+                contracts: [
+                    {
+                        instrumentSymbol: 'ABC-YES',
+                        ticker: 'ABC-YES',
+                        prices: { bestBid: '0.60', bestAsk: '0.62' },
+                    },
+                ],
+            },
+        ],
+        pagination: { total: 1 },
+    };
+
+    const singleEventResponse = {
+        ticker: 'EVT-1',
+        contracts: [
+            {
+                instrumentSymbol: 'ABC-YES',
+                prices: { bestBid: '0.60', bestAsk: '0.62' },
+            },
+        ],
+    };
+
+    function makeGetFetcher(getResponses: unknown[]) {
+        const get = jest.fn(async () => ({ data: getResponses.shift() }));
+        const ctx: FetcherContext = {
+            http: { get } as any,
+            callApi: jest.fn() as any,
+            getHeaders: jest.fn(() => ({})),
+        };
+
+        return { fetcher: new GeminiFetcher(ctx, 'https://api.gemini.test'), get };
+    }
+
+    it('lazily builds the symbol index when fetchRawOrderBook is called first', async () => {
+        // Regression for #2037: a freshly constructed fetcher has an empty
+        // symbolToEventTicker index. fetchRawOrderBook must populate it lazily
+        // instead of throwing when fetchMarkets/fetchEvents was not called first.
+        const { fetcher, get } = makeGetFetcher([eventsResponse, singleEventResponse]);
+
+        const book = await fetcher.fetchRawOrderBook('ABC-YES');
+
+        expect(book).toEqual({
+            bids: [{ price: '0.60', size: '0' }],
+            asks: [{ price: '0.62', size: '0' }],
+            timestamp: expect.any(Number),
+        });
+        // One GET to list events (build the index) + one GET for the single event.
+        expect(get).toHaveBeenCalledTimes(2);
+    });
+
+    it('still throws when the symbol is unknown even after building the index', async () => {
+        const { fetcher } = makeGetFetcher([eventsResponse]);
+
+        await expect(fetcher.fetchRawOrderBook('UNKNOWN-YES')).rejects.toThrow(
+            /no event ticker found for UNKNOWN-YES/,
+        );
+    });
+});