diff --git a/src/bounding_volume/aabb.rs b/src/bounding_volume/aabb.rs
index 35c8f0d8..86b2e42c 100644
--- a/src/bounding_volume/aabb.rs
+++ b/src/bounding_volume/aabb.rs
@@ -994,14 +994,14 @@ impl BoundingVolume for Aabb {
 
     #[inline]
     fn loosen(&mut self, amount: Real) {
-        assert!(amount >= 0.0, "The loosening margin must be positive.");
+        assert!(amount >= 0.0, "The loosening margin must be non-negative.");
         self.mins += Vector::splat(-amount);
         self.maxs += Vector::splat(amount);
     }
 
     #[inline]
     fn loosened(&self, amount: Real) -> Aabb {
-        assert!(amount >= 0.0, "The loosening margin must be positive.");
+        assert!(amount >= 0.0, "The loosening margin must be non-negative.");
         Aabb {
             mins: self.mins + Vector::splat(-amount),
             maxs: self.maxs + Vector::splat(amount),
@@ -1010,7 +1010,7 @@ impl BoundingVolume for Aabb {
 
     #[inline]
     fn tighten(&mut self, amount: Real) {
-        assert!(amount >= 0.0, "The tightening margin must be positive.");
+        assert!(amount >= 0.0, "The tightening margin must be non-negative.");
         self.mins += Vector::splat(amount);
         self.maxs += Vector::splat(-amount);
         assert!(
@@ -1021,7 +1021,7 @@ impl BoundingVolume for Aabb {
 
     #[inline]
     fn tightened(&self, amount: Real) -> Aabb {
-        assert!(amount >= 0.0, "The tightening margin must be positive.");
+        assert!(amount >= 0.0, "The tightening margin must be non-negative.");
 
         Aabb::new(
             self.mins + Vector::splat(amount),
diff --git a/src/bounding_volume/bounding_sphere.rs b/src/bounding_volume/bounding_sphere.rs
index ac9ed1fa..b9b76a61 100644
--- a/src/bounding_volume/bounding_sphere.rs
+++ b/src/bounding_volume/bounding_sphere.rs
@@ -469,7 +469,7 @@ impl BoundingVolume for BoundingSphere {
     /// ```
     #[inline]
     fn loosen(&mut self, amount: Real) {
-        assert!(amount >= 0.0, "The loosening margin must be positive.");
+        assert!(amount >= 0.0, "The loosening margin must be non-negative.");
         self.radius += amount
     }
 
@@ -503,7 +503,7 @@ impl BoundingVolume for BoundingSphere {
     /// ```
     #[inline]
     fn loosened(&self, amount: Real) -> BoundingSphere {
-        assert!(amount >= 0.0, "The loosening margin must be positive.");
+        assert!(amount >= 0.0, "The loosening margin must be non-negative.");
         BoundingSphere::new(self.center, self.radius + amount)
     }
 
@@ -536,7 +536,7 @@ impl BoundingVolume for BoundingSphere {
     /// ```
     #[inline]
     fn tighten(&mut self, amount: Real) {
-        assert!(amount >= 0.0, "The tightening margin must be positive.");
+        assert!(amount >= 0.0, "The tightening margin must be non-negative.");
         assert!(amount <= self.radius, "The tightening margin is to large.");
         self.radius -= amount
     }
@@ -571,7 +571,7 @@ impl BoundingVolume for BoundingSphere {
     /// ```
     #[inline]
     fn tightened(&self, amount: Real) -> BoundingSphere {
-        assert!(amount >= 0.0, "The tightening margin must be positive.");
+        assert!(amount >= 0.0, "The tightening margin must be non-negative.");
         assert!(amount <= self.radius, "The tightening margin is to large.");
         BoundingSphere::new(self.center, self.radius - amount)
     }