diff --git a/Chapter 9/05 - Cython as a language/fibonacci.pyx b/Chapter 9/05 - Cython as a language/fibonacci.pyx
index ffbadf7..b352a00 100644
--- a/Chapter 9/05 - Cython as a language/fibonacci.pyx	
+++ b/Chapter 9/05 - Cython as a language/fibonacci.pyx	
@@ -1,7 +1,7 @@
 """Cython module that provides fibonacci sequence function."""
 
 
-cdef long long fibonacci_cc(unsigned int n):
+cdef long long fibonacci_cc(unsigned int n) nogil:
     if n == 0:
         return 0
     if n == 1: