diff --git a/src/baseApi.ts b/src/baseApi.ts
index a2e36d2..36837ad 100644
--- a/src/baseApi.ts
+++ b/src/baseApi.ts
@@ -194,9 +194,17 @@ export class BaseApi {
       return ''
     }
 
+    // Values must be URL-encoded: cursors are base64 and carry '+', '/' and '=', and an unencoded
+    // '+' reaches the server as a space. Array items are encoded one by one and joined with a
+    // literal comma, which is how the API expects list params such as `ids`.
+    const encode = (value: number | string | boolean): string => encodeURIComponent(String(value))
     const query = Object.keys(params)
       .filter(key => params[key] !== undefined && params[key] !== null)
-      .map(key => key + '=' + params[key])
+      .map(key => {
+        const value = params[key]
+        const encoded = Array.isArray(value) ? value.map(encode).join(',') : encode(value)
+        return `${encodeURIComponent(key)}=${encoded}`
+      })
       .join('&')
     return `?${query}`
   }
diff --git a/tests/transactions.test.ts b/tests/transactions.test.ts
index 98f3025..5d22cc7 100644
--- a/tests/transactions.test.ts
+++ b/tests/transactions.test.ts
@@ -279,3 +279,35 @@ describe('creditCardMetadata deserialization', () => {
     expect(second.billPostDate).toBeNull()
   })
 })
+
+describe('query string encoding', () => {
+  beforeEach(() => {
+    nock.cleanAll()
+    setupAuth()
+  })
+
+  it('URL-encodes a base64 cursor so "+", "/" and "=" reach the API intact', async () => {
+    const cursor = 'MjAyNi0xMC0wNFQxMjowMDowMC4wMDBafGE+Yi9j=='
+    const mock = nock(API_URL)
+      .get('/v2/transactions')
+      .query(actual => actual.after === cursor && actual.accountId === ACCOUNT_ID)
+      .reply(200, mockCursorPage(['tx-1'], null))
+
+    const client = new PluggyClient({ clientId: '123', clientSecret: '456' })
+    await client.fetchTransactionsCursor(ACCOUNT_ID, { after: cursor })
+
+    expect(mock.isDone()).toBeTruthy()
+  })
+
+  it('keeps list params comma-separated', async () => {
+    const mock = nock(API_URL)
+      .get('/v2/transactions')
+      .query(actual => actual.ids === 'a,b' && actual.accountId === ACCOUNT_ID)
+      .reply(200, mockCursorPage([], null))
+
+    const client = new PluggyClient({ clientId: '123', clientSecret: '456' })
+    await client.fetchTransactionsCursor(ACCOUNT_ID, { ids: ['a', 'b'] })
+
+    expect(mock.isDone()).toBeTruthy()
+  })
+})