diff --git a/pkg/constraint/constraint.go b/pkg/constraint/constraint.go
index 8acc8c073d8..da0c3749e14 100644
--- a/pkg/constraint/constraint.go
+++ b/pkg/constraint/constraint.go
@@ -17,11 +17,16 @@ limitations under the License.
 package constraint
 
 import (
+	"fmt"
+
 	"github.com/GoogleCloudPlatform/kubernetes/pkg/api"
 )
 
 // Allowed returns true if pods is a collection of bound pods
 // which can run without conflict on a single minion.
-func Allowed(pods []api.BoundPod) bool {
-	return !PortsConflict(pods)
+func Allowed(pods []api.BoundPod) error {
+	if (PortsConflict(pods)) {
+		return fmt.Errorf("conflicting ports");
+	}
+	return nil;
 }
diff --git a/pkg/constraint/constraint_test.go b/pkg/constraint/constraint_test.go
index 1677d935df5..530428f91cc 100644
--- a/pkg/constraint/constraint_test.go
+++ b/pkg/constraint/constraint_test.go
@@ -17,6 +17,7 @@ limitations under the License.
 package constraint
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/GoogleCloudPlatform/kubernetes/pkg/api"
@@ -40,11 +41,11 @@ func podWithContainers(containers ...api.Container) api.BoundPod {
 
 func TestAllowed(t *testing.T) {
 	table := []struct {
-		allowed bool
+		err	error
 		pods    []api.BoundPod
 	}{
 		{
-			allowed: true,
+			err: nil,
 			pods: []api.BoundPod{
 				podWithContainers(
 					containerWithHostPorts(1, 2, 3),
@@ -57,7 +58,7 @@ func TestAllowed(t *testing.T) {
 			},
 		},
 		{
-			allowed: true,
+			err: nil,
 			pods: []api.BoundPod{
 				podWithContainers(
 					containerWithHostPorts(0, 0),
@@ -70,7 +71,7 @@ func TestAllowed(t *testing.T) {
 			},
 		},
 		{
-			allowed: false,
+			err: fmt.Errorf("conflicting ports"),
 			pods: []api.BoundPod{
 				podWithContainers(
 					containerWithHostPorts(3, 3),
@@ -78,7 +79,7 @@ func TestAllowed(t *testing.T) {
 			},
 		},
 		{
-			allowed: false,
+			err: fmt.Errorf("conflicting ports"),
 			pods: []api.BoundPod{
 				podWithContainers(
 					containerWithHostPorts(6),
@@ -91,7 +92,7 @@ func TestAllowed(t *testing.T) {
 	}
 
 	for _, item := range table {
-		if e, a := item.allowed, Allowed(item.pods); e != a {
+		if e, a := item.err, Allowed(item.pods); e != a && e.Error() != a.Error() {
 			t.Errorf("Expected %v, got %v: \n%v\v", e, a, item.pods)
 		}
 	}
diff --git a/pkg/registry/etcd/etcd.go b/pkg/registry/etcd/etcd.go
index d2bd576cb52..49c32eff69e 100644
--- a/pkg/registry/etcd/etcd.go
+++ b/pkg/registry/etcd/etcd.go
@@ -214,8 +214,8 @@ func (r *Registry) assignPod(ctx api.Context, podID string, machine string) erro
 	err = r.AtomicUpdate(contKey, &api.BoundPods{}, func(in runtime.Object) (runtime.Object, error) {
 		boundPodList := in.(*api.BoundPods)
 		boundPodList.Items = append(boundPodList.Items, *boundPod)
-		if !constraint.Allowed(boundPodList.Items) {
-			return nil, fmt.Errorf("the assignment would cause a constraint violation")
+		if e := constraint.Allowed(boundPodList.Items); e != nil {
+			return nil, fmt.Errorf("the assignment would cause the following constraint violation: %v", e)
 		}
 		return boundPodList, nil
 	})