diff --git a/examples/cart_pole/cartpole_state_est.m b/examples/cart_pole/cartpole_state_est.m
index da73d49..df3cd77 100644
--- a/examples/cart_pole/cartpole_state_est.m
+++ b/examples/cart_pole/cartpole_state_est.m
@@ -3,7 +3,7 @@
 
 function retval = cartpole_state_est(true_x)
 
-    xdim = size(true_x)(1);
+    xdim = size(true_x, 1);
     H = eye(xdim);
     retval = H * true_x;