diff --git a/x-pack/legacy/plugins/apm/public/components/shared/ErrorRateAlertTrigger/index.tsx b/x-pack/legacy/plugins/apm/public/components/shared/ErrorRateAlertTrigger/index.tsx
index 6d0a2b96092a..9bfc5936a555 100644
--- a/x-pack/legacy/plugins/apm/public/components/shared/ErrorRateAlertTrigger/index.tsx
+++ b/x-pack/legacy/plugins/apm/public/components/shared/ErrorRateAlertTrigger/index.tsx
@@ -58,7 +58,7 @@ export function ErrorRateAlertTrigger(props: Props) {
     </PopoverExpression>,
     <ForLastExpression
       onChangeWindowSize={windowSize =>
-        setAlertParams('windowSize', windowSize)
+        setAlertParams('windowSize', windowSize || '')
       }
       onChangeWindowUnit={windowUnit =>
         setAlertParams('windowUnit', windowUnit)
diff --git a/x-pack/legacy/plugins/apm/public/components/shared/TransactionDurationAlertTrigger/index.tsx b/x-pack/legacy/plugins/apm/public/components/shared/TransactionDurationAlertTrigger/index.tsx
index cdc7c30089b4..077e6535a8b2 100644
--- a/x-pack/legacy/plugins/apm/public/components/shared/TransactionDurationAlertTrigger/index.tsx
+++ b/x-pack/legacy/plugins/apm/public/components/shared/TransactionDurationAlertTrigger/index.tsx
@@ -123,7 +123,7 @@ export function TransactionDurationAlertTrigger(props: Props) {
     </PopoverExpression>,
     <ForLastExpression
       onChangeWindowSize={timeWindowSize =>
-        setAlertParams('windowSize', timeWindowSize)
+        setAlertParams('windowSize', timeWindowSize || '')
       }
       onChangeWindowUnit={timeWindowUnit =>
         setAlertParams('windowUnit', timeWindowUnit)