diff --git a/GPflowOpt/optim.py b/GPflowOpt/optim.py
index 955ff5b..15dfe49 100644
--- a/GPflowOpt/optim.py
+++ b/GPflowOpt/optim.py
@@ -129,7 +129,8 @@ def __init__(self, domain, method='L-BFGS-B', tol=None, maxiter=1000):
                            options=options)
 
     def _optimize(self, objective):
-        result = minimize(fun=objective,
+        objective1d = lambda X: tuple(map(lambda arr: arr.ravel(), objective(X)))
+        result = minimize(fun=objective1d,
                           x0=self.get_initial(),
                           jac=self.gradient_enabled(),
                           bounds=list(zip(self.domain.lower, self.domain.upper)),