diff --git a/numpy_questions.py b/numpy_questions.py
index 07a10c1..a483924 100644
--- a/numpy_questions.py
+++ b/numpy_questions.py
@@ -41,7 +41,13 @@ def max_index(X):
     j = 0
 
     # TODO
+    if not isinstance(X, np.ndarray):
+        raise ValueError("Input must be a numpy array")
 
+    if X.ndim != 2:
+        raise ValueError("Input must be 2D")
+
+    i, j = np.unravel_index(np.argmax(X, axis=None), X.shape)
     return i, j
 
 
@@ -64,4 +70,10 @@ 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.
+
+    product = 1
+    for n in range(1, n_terms+1):
+        product *= (4 * n**2) / (4 * n**2 - 1)
+
+    pi = 2 * product
+    return pi