diff --git a/src/structs/mat.rs b/src/structs/mat.rs
index 40604e9b..70321bb2 100644
--- a/src/structs/mat.rs
+++ b/src/structs/mat.rs
@@ -19,6 +19,7 @@ use traits::geometry::{ToHomogeneous, FromHomogeneous, Orig};
 use linalg;
 #[cfg(feature="arbitrary")]
 use quickcheck::{Arbitrary, Gen};
+use std::fmt::Debug;
 
 
 /// Special identity matrix. All its operation are no-ops.
diff --git a/src/structs/mat_macros.rs b/src/structs/mat_macros.rs
index d95ffd2d..7b1d3263 100644
--- a/src/structs/mat_macros.rs
+++ b/src/structs/mat_macros.rs
@@ -702,7 +702,7 @@ macro_rules! from_homogeneous_impl(
 
 macro_rules! outer_impl(
     ($t: ident, $m: ident) => (
-        impl<N: Copy + Mul<N, Output = N> + Zero> Outer for $t<N> {
+        impl<N: Copy + Debug + Mul<N, Output = N> + Zero> Outer for $t<N> {
             type OuterProductType = $m<N>;
 
             #[inline]
diff --git a/src/traits/operations.rs b/src/traits/operations.rs
index 80c899ce..8b737efe 100644
--- a/src/traits/operations.rs
+++ b/src/traits/operations.rs
@@ -4,6 +4,7 @@ use num::{Float, Signed};
 use std::ops::Mul;
 use std::cmp::Ordering;
 use traits::structure::SquareMat;
+use std::fmt::Debug;
 
 /// Result of a partial ordering.
 #[derive(Eq, PartialEq, RustcEncodable, RustcDecodable, Clone, Debug, Copy)]
@@ -300,9 +301,9 @@ pub trait Transpose {
 }
 
 /// Traits of objects having an outer product.
-pub trait Outer {
+pub trait Outer : Debug {
     /// Result type of the outer product.
-    type OuterProductType;
+    type OuterProductType : Debug;
 
     /// Computes the outer product: `a * b`
     fn outer(&self, other: &Self) -> Self::OuterProductType;