diff --git a/src/bounding_volume/aabb.rs b/src/bounding_volume/aabb.rs
index 2fcce281..1a1d0b13 100644
--- a/src/bounding_volume/aabb.rs
+++ b/src/bounding_volume/aabb.rs
@@ -973,14 +973,14 @@ 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::repeat(-amount);
         self.maxs += Vector::repeat(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::repeat(-amount),
             maxs: self.maxs + Vector::repeat(amount),
@@ -989,7 +989,7 @@ 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::repeat(amount);
         self.maxs += Vector::repeat(-amount);
         assert!(
@@ -1000,7 +1000,7 @@ 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::repeat(amount),
diff --git a/src/bounding_volume/bounding_sphere.rs b/src/bounding_volume/bounding_sphere.rs
index 77420a8a..ad0ad942 100644
--- a/src/bounding_volume/bounding_sphere.rs
+++ b/src/bounding_volume/bounding_sphere.rs
@@ -475,7 +475,7 @@ 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
     }
 
@@ -509,7 +509,7 @@ 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)
     }
 
@@ -542,7 +542,7 @@ 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
     }
@@ -577,7 +577,7 @@ 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)
     }