diff --git a/ui/effects/animation_value.cpp b/ui/effects/animation_value.cpp
index 6b6b299..934d9c8 100644
--- a/ui/effects/animation_value.cpp
+++ b/ui/effects/animation_value.cpp
@@ -19,7 +19,12 @@ int SlowMultiplierMinusOne/* = 0*/;
 } // namespace
 
 transition linear = [](const float64 &delta, const float64 &dt) {
-	return delta * dt;
+	Expects(!std::isnan(delta));
+	Expects(!std::isnan(dt));
+
+	const auto result = delta * dt;
+	Ensures(!std::isnan(result));
+	return result;
 };
 
 transition sineInOut = [](const float64 &delta, const float64 &dt) {
diff --git a/ui/effects/animations.h b/ui/effects/animations.h
index e7fd974..bdc3a45 100644
--- a/ui/effects/animations.h
+++ b/ui/effects/animations.h
@@ -345,9 +345,12 @@ inline void Simple::start(
 		Assert(!std::isnan(that->delta));
 		Assert(!std::isnan(that->duration));
 		const auto finished = (time >= that->duration);
+		Assert(finished || that->duration > 0);
+		const auto progressRatio = finished ? 1. : time / that->duration;
+		Assert(!std::isnan(progressRatio));
 		const auto progress = finished
 			? that->delta
-			: that->transition(that->delta, time / that->duration);
+			: that->transition(that->delta, progressRatio);
 		Assert(!std::isnan(that->from));
 		Assert(!std::isnan(progress));
 		that->value = that->from + progress;