diff --git a/CHANGELOG.md b/CHANGELOG.md
index f9de4cb5a..109beacbb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Upgraded `fuse.js` from version `7.3.0` to `7.5.0`
 
+### Fixed
+
+- Resolved an exception in the `POST api/v1/activities` endpoint when creating an activity with the update account balance option but without an account
+
 ## 3.33.0 - 2026-07-25
 
 ### Added
diff --git a/apps/api/src/app/account/account.service.ts b/apps/api/src/app/account/account.service.ts
index f84f085a3..209806234 100644
--- a/apps/api/src/app/account/account.service.ts
+++ b/apps/api/src/app/account/account.service.ts
@@ -37,11 +37,26 @@ export class AccountService {
   public async account({
     id_userId
   }: Prisma.AccountWhereUniqueInput): Promise<Account | null> {
-    const [account] = await this.accounts({
-      where: id_userId
+    const account = await this.prismaService.account.findUnique({
+      include: {
+        balances: {
+          orderBy: { date: 'desc' },
+          take: 1
+        }
+      },
+      where: { id_userId }
     });
 
-    return account;
+    if (!account) {
+      return null;
+    }
+
+    const { balances, ...accountData } = account;
+
+    return {
+      ...accountData,
+      balance: balances[0]?.value ?? 0
+    };
   }
 
   public async accountWithActivities(
diff --git a/apps/api/src/app/activities/activities.service.ts b/apps/api/src/app/activities/activities.service.ts
index 459293abd..fbe93d9a0 100644
--- a/apps/api/src/app/activities/activities.service.ts
+++ b/apps/api/src/app/activities/activities.service.ts
@@ -275,7 +275,7 @@ export class ActivitiesService {
       include: { SymbolProfile: true }
     });
 
-    if (updateAccountBalance === true) {
+    if (accountId && updateAccountBalance === true) {
       let amount = new Big(data.unitPrice).mul(data.quantity);
 
       if (['BUY', 'FEE'].includes(data.type)) {
diff --git a/apps/client/src/app/pages/portfolio/activities/create-or-update-activity-dialog/create-or-update-activity-dialog.component.ts b/apps/client/src/app/pages/portfolio/activities/create-or-update-activity-dialog/create-or-update-activity-dialog.component.ts
index 79e1e8983..632db1cd4 100644
--- a/apps/client/src/app/pages/portfolio/activities/create-or-update-activity-dialog/create-or-update-activity-dialog.component.ts
+++ b/apps/client/src/app/pages/portfolio/activities/create-or-update-activity-dialog/create-or-update-activity-dialog.component.ts
@@ -266,16 +266,9 @@ export class GfCreateOrUpdateActivityDialogComponent {
 
         this.activityForm.get('currency')?.setValue(currency);
         this.activityForm.get('currencyOfUnitPrice')?.setValue(currency);
-
-        if (['FEE', 'INTEREST'].includes(type)) {
-          if (this.activityForm.get('accountId')?.value) {
-            this.activityForm.get('updateAccountBalance')?.enable();
-          } else {
-            this.activityForm.get('updateAccountBalance')?.disable();
-            this.activityForm.get('updateAccountBalance')?.setValue(false);
-          }
-        }
       }
+
+      this.syncUpdateAccountBalanceControl();
     });
 
     this.activityForm
@@ -299,12 +292,7 @@ export class GfCreateOrUpdateActivityDialogComponent {
       });
 
     this.activityForm.get('date')?.valueChanges.subscribe(() => {
-      if (isToday(this.activityForm.get('date')?.value)) {
-        this.activityForm.get('updateAccountBalance')?.enable();
-      } else {
-        this.activityForm.get('updateAccountBalance')?.disable();
-        this.activityForm.get('updateAccountBalance')?.setValue(false);
-      }
+      this.syncUpdateAccountBalanceControl();
 
       this.changeDetectorRef.markForCheck();
     });
@@ -384,8 +372,6 @@ export class GfCreateOrUpdateActivityDialogComponent {
             .get('searchSymbol')
             ?.removeValidators(Validators.required);
           this.activityForm.get('searchSymbol')?.updateValueAndValidity();
-          this.activityForm.get('updateAccountBalance')?.disable();
-          this.activityForm.get('updateAccountBalance')?.setValue(false);
         } else if (['FEE', 'INTEREST', 'LIABILITY'].includes(type)) {
           const currency =
             this.data.accounts.find(({ id }) => {
@@ -421,16 +407,6 @@ export class GfCreateOrUpdateActivityDialogComponent {
           if (type === 'FEE') {
             this.activityForm.get('unitPrice')?.setValue(0);
           }
-
-          if (
-            ['FEE', 'INTEREST'].includes(type) &&
-            this.activityForm.get('accountId')?.value
-          ) {
-            this.activityForm.get('updateAccountBalance')?.enable();
-          } else {
-            this.activityForm.get('updateAccountBalance')?.disable();
-            this.activityForm.get('updateAccountBalance')?.setValue(false);
-          }
         } else {
           this.activityForm
             .get('dataSource')
@@ -442,9 +418,10 @@ export class GfCreateOrUpdateActivityDialogComponent {
             .get('searchSymbol')
             ?.setValidators(Validators.required);
           this.activityForm.get('searchSymbol')?.updateValueAndValidity();
-          this.activityForm.get('updateAccountBalance')?.enable();
         }
 
+        this.syncUpdateAccountBalanceControl();
+
         this.changeDetectorRef.markForCheck();
       });
 
@@ -559,6 +536,27 @@ export class GfCreateOrUpdateActivityDialogComponent {
     }
   }
 
+  private syncUpdateAccountBalanceControl() {
+    const accountBalanceControl = this.activityForm.get('updateAccountBalance');
+    const accountId = this.activityForm.get('accountId')?.value;
+    const dataSource = this.activityForm.get('dataSource')?.value;
+    const date = this.activityForm.get('date')?.value;
+    const type = this.activityForm.get('type')?.value;
+
+    const isEligible =
+      !!accountId &&
+      isToday(date) &&
+      !['LIABILITY', 'VALUABLE'].includes(type) &&
+      !(dataSource === 'MANUAL' && type === 'BUY');
+
+    if (isEligible) {
+      accountBalanceControl?.enable();
+    } else {
+      accountBalanceControl?.disable();
+      accountBalanceControl?.setValue(false);
+    }
+  }
+
   private updateAssetProfile() {
     this.isLoading = true;
     this.changeDetectorRef.markForCheck();