diff --git a/src/Core/Billing/Organizations/Commands/PreviewOrganizationTaxCommand.cs b/src/Core/Billing/Organizations/Commands/PreviewOrganizationTaxCommand.cs
index 747b993cce3a..72164d3f2802 100644
--- a/src/Core/Billing/Organizations/Commands/PreviewOrganizationTaxCommand.cs
+++ b/src/Core/Billing/Organizations/Commands/PreviewOrganizationTaxCommand.cs
@@ -45,6 +45,8 @@ public class PreviewOrganizationTaxCommand(
     ISubscriptionDiscountService subscriptionDiscountService)
     : BaseBillingCommand<PreviewOrganizationTaxCommand>(logger), IPreviewOrganizationTaxCommand
 {
+    private readonly ILogger<PreviewOrganizationTaxCommand> _logger = logger;
+
     public Task<BillingCommandResult<(decimal Tax, decimal Total)>> Run(
         User user,
         OrganizationSubscriptionPurchase purchase,
@@ -231,15 +233,34 @@ public class PreviewOrganizationTaxCommand(
 
                 var items = new List<InvoiceSubscriptionDetailsItemOptions>();
 
-                var passwordManagerSeats = subscriptionItemsByPriceId[
-                    currentPlan.HasNonSeatBasedPasswordManagerPlan()
+                long quantity;
+
+                if (currentPlan.HasNonSeatBasedPasswordManagerPlan() && !newPlan.HasNonSeatBasedPasswordManagerPlan())
+                {
+                    // The current plan doesn't have a per-seat subscription item to read a quantity from
+                    // (e.g. upgrading from a flat-rate plan like Teams Starter), so fall back to the
+                    // organization's occupied seat count instead of looking it up on the subscription.
+                    quantity = (long)organization.Seats!;
+                }
+                else
+                {
+                    var passwordManagerPriceId = currentPlan.HasNonSeatBasedPasswordManagerPlan()
                         ? currentPlan.PasswordManager.StripePlanId
-                        : currentPlan.PasswordManager.StripeSeatPlanId];
+                        : currentPlan.PasswordManager.StripeSeatPlanId;
 
-                var quantity = currentPlan.HasNonSeatBasedPasswordManagerPlan() &&
-                               !newPlan.HasNonSeatBasedPasswordManagerPlan()
-                    ? (long)organization.Seats!
-                    : passwordManagerSeats.Quantity;
+                    if (!subscriptionItemsByPriceId.TryGetValue(passwordManagerPriceId, out var passwordManagerSeats))
+                    {
+                        _logger.LogError(
+                            "Organization {OrganizationId}'s subscription {SubscriptionId} does not contain a " +
+                            "Password Manager line item matching its current plan's price {PriceId}",
+                            organization.Id, organization.GatewaySubscriptionId, passwordManagerPriceId);
+
+                        return new BadRequest(
+                            "Your organization's subscription does not match its current plan. Please contact support for assistance.");
+                    }
+
+                    quantity = passwordManagerSeats.Quantity;
+                }
 
                 items.Add(new InvoiceSubscriptionDetailsItemOptions
                 {
diff --git a/test/Core.Test/Billing/Organizations/Commands/PreviewOrganizationTaxCommandTests.cs b/test/Core.Test/Billing/Organizations/Commands/PreviewOrganizationTaxCommandTests.cs
index ad5ca2390e81..b5f28210c828 100644
--- a/test/Core.Test/Billing/Organizations/Commands/PreviewOrganizationTaxCommandTests.cs
+++ b/test/Core.Test/Billing/Organizations/Commands/PreviewOrganizationTaxCommandTests.cs
@@ -1494,6 +1494,132 @@ await _stripeAdapter.Received(1).CreateInvoicePreviewAsync(Arg.Is<InvoiceCreateP
                 item.Price == plan.SecretsManager.StripeServiceAccountPlanId && item.Quantity == 20)));
     }
 
+    // Regression test: upgrading from a flat-rate, non-seat-based plan (e.g. Teams Starter) has no
+    // per-seat Password Manager line item on the subscription to read a quantity from, so the command
+    // must fall back to the organization's occupied seat count instead of throwing a KeyNotFoundException.
+    [Fact]
+    public async Task Run_OrganizationPlanChange_TeamsStarterToTeams_UsesOrganizationSeats()
+    {
+        var organization = new Organization
+        {
+            Id = Guid.NewGuid(),
+            PlanType = PlanType.TeamsStarter2023,
+            GatewayCustomerId = "cus_test123",
+            GatewaySubscriptionId = "sub_test123",
+            UseSecretsManager = false,
+            Seats = 10
+        };
+
+        var planChange = new OrganizationSubscriptionPlanChange
+        {
+            Tier = ProductTierType.Teams,
+            Cadence = PlanCadenceType.Annually
+        };
+
+        var billingAddress = new BillingAddress
+        {
+            Country = "US",
+            PostalCode = "10012"
+        };
+
+        var currentPlan = new TeamsStarterPlan2023();
+        var newPlan = new Teams2023Plan(true);
+        _pricingClient.GetPlanOrThrow(organization.PlanType).Returns(currentPlan);
+        _pricingClient.GetPlanOrThrow(planChange.PlanType).Returns(newPlan);
+
+        // The flat-rate plan's subscription only contains its single, non-seat-based line item -
+        // there is no per-seat price for the command to look up.
+        var subscriptionItems = new List<SubscriptionItem>
+        {
+            new() { Price = new Price { Id = currentPlan.PasswordManager.StripePlanId }, Quantity = 1 }
+        };
+
+        var subscription = new Subscription
+        {
+            Id = "sub_test123",
+            Items = new StripeList<SubscriptionItem> { Data = subscriptionItems },
+            Customer = new Customer { Discount = null }
+        };
+
+        _stripeAdapter.GetSubscriptionAsync("sub_test123", Arg.Any<SubscriptionGetOptions>()).Returns(subscription);
+
+        var invoice = new Invoice
+        {
+            TotalTaxes = [new InvoiceTotalTax { Amount = 900 }],
+            Total = 9900
+        };
+
+        _stripeAdapter.CreateInvoicePreviewAsync(Arg.Any<InvoiceCreatePreviewOptions>()).Returns(invoice);
+
+        var result = await _command.Run(organization, planChange, billingAddress);
+
+        Assert.True(result.IsT0);
+
+        await _stripeAdapter.Received(1).CreateInvoicePreviewAsync(Arg.Is<InvoiceCreatePreviewOptions>(options =>
+            options.SubscriptionDetails.Items.Count == 1 &&
+            options.SubscriptionDetails.Items[0].Price == newPlan.PasswordManager.StripeSeatPlanId &&
+            options.SubscriptionDetails.Items[0].Quantity == organization.Seats));
+    }
+
+    // Regression test: if the organization's current plan is seat-based but its subscription does not
+    // contain a line item matching that plan's price (a data discrepancy), the command should return a
+    // BadRequest rather than throwing a KeyNotFoundException from the dictionary lookup.
+    [Fact]
+    public async Task Run_OrganizationPlanChange_SubscriptionMissingCurrentPlanPriceLineItem_ReturnsBadRequest()
+    {
+        var organization = new Organization
+        {
+            Id = Guid.NewGuid(),
+            PlanType = PlanType.TeamsMonthly2023,
+            GatewayCustomerId = "cus_test123",
+            GatewaySubscriptionId = "sub_test123",
+            UseSecretsManager = false,
+            Seats = 5
+        };
+
+        var planChange = new OrganizationSubscriptionPlanChange
+        {
+            Tier = ProductTierType.Enterprise,
+            Cadence = PlanCadenceType.Annually
+        };
+
+        var billingAddress = new BillingAddress
+        {
+            Country = "US",
+            PostalCode = "12345"
+        };
+
+        var currentPlan = new Teams2023Plan(false);
+        var newPlan = new EnterprisePlan(true);
+        _pricingClient.GetPlanOrThrow(organization.PlanType).Returns(currentPlan);
+        _pricingClient.GetPlanOrThrow(planChange.PlanType).Returns(newPlan);
+
+        // The subscription does not contain a line item for the current plan's Password Manager price.
+        var subscriptionItems = new List<SubscriptionItem>
+        {
+            new() { Price = new Price { Id = "some-other-unrelated-price" }, Quantity = 5 }
+        };
+
+        var subscription = new Subscription
+        {
+            Id = "sub_test123",
+            Items = new StripeList<SubscriptionItem> { Data = subscriptionItems },
+            Customer = new Customer { Discount = null }
+        };
+
+        _stripeAdapter.GetSubscriptionAsync("sub_test123", Arg.Any<SubscriptionGetOptions>()).Returns(subscription);
+
+        var result = await _command.Run(organization, planChange, billingAddress);
+
+        Assert.True(result.IsT1);
+        var badRequest = result.AsT1;
+        Assert.Equal(
+            "Your organization's subscription does not match its current plan. Please contact support for assistance.",
+            badRequest.Response);
+
+        await _stripeAdapter.DidNotReceive().CreateInvoicePreviewAsync(Arg.Any<InvoiceCreatePreviewOptions>());
+    }
+
     [Fact]
     public async Task Run_OrganizationPlanChange_ExistingSubscriptionWithDiscount_PreservesCoupon()
     {