diff --git a/internal/services/appservice/migration/windows_function_app_slot.go b/internal/services/appservice/migration/windows_function_app_slot.go
index b73f321481ca..ba9c67fee85c 100644
--- a/internal/services/appservice/migration/windows_function_app_slot.go
+++ b/internal/services/appservice/migration/windows_function_app_slot.go
@@ -1527,9 +1527,9 @@ func (w WindowsFunctionAppSlotV0toV1) Schema() map[string]*pluginsdk.Schema {
 
 func (w WindowsFunctionAppSlotV0toV1) UpgradeFunc() pluginsdk.StateUpgraderFunc {
 	return func(ctx context.Context, rawState map[string]interface{}, meta interface{}) (map[string]interface{}, error) {
-		oldId := rawState["service_plan_id"].(string)
+		oldId, ok := rawState["service_plan_id"].(string)
 		// service_plan_id can be empty if it is not in a different Service Plan to the "parent" app
-		if oldId == "" {
+		if !ok || oldId == "" {
 			return rawState, nil
 		}
 		parsedId, err := commonids.ParseAppServicePlanIDInsensitively(oldId)