diff --git a/numpy_questions.py b/numpy_questions.py
index 07a10c1..94ffec6 100644
--- a/numpy_questions.py
+++ b/numpy_questions.py
@@ -41,7 +41,9 @@ def max_index(X):
     j = 0
 
     # TODO
-
+    if X is None or not isinstance(X, np.ndarray):
+        raise ValueError("Input must be a numpy array.")
+    i, j = np.unravel_index(np.argmax(X), X.shape)
     return i, j
 
 
@@ -64,4 +66,9 @@ def wallis_product(n_terms):
     """
     # XXX : The n_terms is an int that corresponds to the number of
     # terms in the product. For example 10000.
-    return 0.
+    if n_terms == 0:
+        return 2.
+    prod = 1.
+    for n in range(1, n_terms+1):
+        prod = prod * (4 * n ** 2) / ((2 * n - 1)*(2 * n + 1))
+    return 2*prod