diff --git a/packages/cubejs-client-core/src/index.ts b/packages/cubejs-client-core/src/index.ts
index 1bac5d7708989..3dc2f9907625c 100644
--- a/packages/cubejs-client-core/src/index.ts
+++ b/packages/cubejs-client-core/src/index.ts
@@ -786,19 +786,47 @@ class CubeApi {
 
         const [schema, ...data] = response.error.split('\n');
 
+        let parsedSchema: any;
         try {
-          const parsedSchema = JSON.parse(schema);
-          return {
-            schema: parsedSchema.schema,
-            data: data
-              .filter((d: string) => d.trim().length)
-              .map((d: string) => JSON.parse(d).data)
-              .reduce((a: any, b: any) => a.concat(b), []),
-            ...(parsedSchema.lastRefreshTime ? { lastRefreshTime: parsedSchema.lastRefreshTime } : {}),
-          };
+          parsedSchema = JSON.parse(schema);
         } catch (err) {
+          // Schema line isn't valid JSON — the whole `error` payload is a real error.
           throw new Error(response.error);
         }
+
+        const rows: any[] = [];
+
+        for (const line of data) {
+          if (line.trim().length) {
+            let parsed: any;
+            try {
+              parsed = JSON.parse(line);
+            } catch (err) {
+              // A non-JSON line after a valid schema means a malformed payload — fall
+              // back to surfacing the raw response rather than dropping rows silently.
+              throw new Error(response.error);
+            }
+
+            // The stream can interleave an error chunk after the schema (e.g. a
+            // post-processing/cast error surfaced mid-result). Such a line has no
+            // `data`, so the previous `JSON.parse(d).data` concat pushed an `undefined`
+            // "phantom" row and silently swallowed the failure. Surface it instead —
+            // matching how `cubeSqlStream` classifies `error` chunks.
+            if (parsed.error) {
+              throw new Error(parsed.error);
+            }
+
+            if (parsed.data) {
+              rows.push(...parsed.data);
+            }
+          }
+        }
+
+        return {
+          schema: parsedSchema.schema,
+          data: rows,
+          ...(parsedSchema.lastRefreshTime ? { lastRefreshTime: parsedSchema.lastRefreshTime } : {}),
+        };
       },
       options,
       callback
diff --git a/packages/cubejs-client-core/test/CubeApi.test.ts b/packages/cubejs-client-core/test/CubeApi.test.ts
index 8819bff257100..8560ade1c3dff 100644
--- a/packages/cubejs-client-core/test/CubeApi.test.ts
+++ b/packages/cubejs-client-core/test/CubeApi.test.ts
@@ -427,6 +427,21 @@ describe('CubeApi cubeSql', () => {
     JSON.stringify({ data: [['Active']] }),
   ].join('\n');
 
+  // The backend streams a schema chunk, then (on a post-processing failure) an error
+  // chunk. The error must surface as a rejection instead of being concatenated as an
+  // `undefined` phantom row.
+  const cubeSqlResponseBodyWithError = [
+    JSON.stringify({
+      schema: [
+        { name: 'created_date', column_type: 'String' },
+      ],
+    }),
+    JSON.stringify({
+      error: 'Post-Processing Error: Cast error: Error parsing \'2026-05-01\' as timestamp',
+      requestId: '2fbe44e4-df6f-420d-ae39-376c802323b4-span-1',
+    }),
+  ].join('\n');
+
   test('should parse lastRefreshTime from response', async () => {
     vi.spyOn(HttpTransport.prototype, 'request').mockImplementation(() => ({
       subscribe: (cb) => Promise.resolve(cb({
@@ -471,6 +486,24 @@ describe('CubeApi cubeSql', () => {
     expect(res.schema).toEqual([{ name: 'status', column_type: 'String' }]);
     expect(res.data).toEqual([['Active']]);
   });
+
+  test('should surface an error chunk that follows the schema instead of swallowing it', async () => {
+    vi.spyOn(HttpTransport.prototype, 'request').mockImplementation(() => ({
+      subscribe: (cb) => Promise.resolve(cb({
+        status: 200,
+        text: () => Promise.resolve(JSON.stringify({ error: cubeSqlResponseBodyWithError })),
+      } as any,
+      async () => undefined as any))
+    }));
+
+    const cubeApi = new CubeApi('token', {
+      apiUrl: 'http://localhost:4000/cubejs-api/v1',
+    });
+
+    await expect(
+      cubeApi.cubeSql('SELECT created_date FROM deals')
+    ).rejects.toThrow('Post-Processing Error: Cast error: Error parsing \'2026-05-01\' as timestamp');
+  });
 });
 
 describe('CubeApi with baseRequestId', () => {