diff --git a/lectures/short_path.md b/lectures/short_path.md
index bd49a06f8..0ae0ebdd1 100644
--- a/lectures/short_path.md
+++ b/lectures/short_path.md
@@ -428,6 +428,7 @@ def bellman(J, Q):
 
 
 def compute_cost_to_go(Q):
+    num_nodes = Q.shape[0]
     J = np.zeros(num_nodes)      # Initial guess
     next_J = np.empty(num_nodes)  # Stores updated guess
     max_iter = 500