diff --git a/lib/icinga/downtime.cpp b/lib/icinga/downtime.cpp
index 23afdbef4..8f4638526 100644
--- a/lib/icinga/downtime.cpp
+++ b/lib/icinga/downtime.cpp
@@ -584,6 +584,14 @@ void Downtime::ValidateEndTime(const Lazy<Timestamp>& lvalue, const ValidationUt
 		BOOST_THROW_EXCEPTION(ValidationError(this, { "end_time" }, "End time must be greater than 0."));
 }
 
+void Downtime::ValidateDuration(const Lazy<Timestamp>& lvalue, const ValidationUtils& utils)
+{
+	ObjectImpl<Downtime>::ValidateDuration(lvalue, utils);
+
+	if (lvalue() < 0)
+		BOOST_THROW_EXCEPTION(ValidationError(this, { "duration" }, "Duration must be positive."));
+}
+
 DowntimeChildOptions Downtime::ChildOptionsFromValue(const Value& options)
 {
 	if (options == "DowntimeNoChildren")
diff --git a/lib/icinga/downtime.hpp b/lib/icinga/downtime.hpp
index 519d78f2b..d9e65fc16 100644
--- a/lib/icinga/downtime.hpp
+++ b/lib/icinga/downtime.hpp
@@ -84,6 +84,7 @@ protected:
 
 	void ValidateStartTime(const Lazy<Timestamp>& lvalue, const ValidationUtils& utils) override;
 	void ValidateEndTime(const Lazy<Timestamp>& lvalue, const ValidationUtils& utils) override;
+	void ValidateDuration(const Lazy<Timestamp>& lvalue, const ValidationUtils& utils) override;
 
 private:
 	ObjectImpl<Checkable>::Ptr m_Checkable;
diff --git a/lib/icinga/scheduleddowntime.cpp b/lib/icinga/scheduleddowntime.cpp
index 6afdf602c..87b23865d 100644
--- a/lib/icinga/scheduleddowntime.cpp
+++ b/lib/icinga/scheduleddowntime.cpp
@@ -89,6 +89,14 @@ void ScheduledDowntime::Start(bool runtimeCreated)
 		Utility::QueueAsyncCallback([this]() { CreateNextDowntime(); });
 }
 
+void ScheduledDowntime::ValidateDuration(const Lazy<Timestamp>& lvalue, const ValidationUtils& utils)
+{
+	ObjectImpl<ScheduledDowntime>::ValidateDuration(lvalue, utils);
+
+	if (lvalue() < 0)
+		BOOST_THROW_EXCEPTION(ValidationError(this, { "duration" }, "Duration must be positive."));
+}
+
 void ScheduledDowntime::TimerProc()
 {
 	for (const ScheduledDowntime::Ptr& sd : ConfigType::GetObjectsByType<ScheduledDowntime>()) {
diff --git a/lib/icinga/scheduleddowntime.hpp b/lib/icinga/scheduleddowntime.hpp
index 8ede9666c..fc65c99b2 100644
--- a/lib/icinga/scheduleddowntime.hpp
+++ b/lib/icinga/scheduleddowntime.hpp
@@ -41,6 +41,7 @@ public:
 protected:
 	void OnAllConfigLoaded() override;
 	void Start(bool runtimeCreated) override;
+	void ValidateDuration(const Lazy<Timestamp>& lvalue, const ValidationUtils& utils) override;
 
 private:
 	static void TimerProc();