diff --git a/cmfrec/cfuns_double.pyx b/cmfrec/cfuns_double.pyx
index 544456e..a3f7c9d 100644
--- a/cmfrec/cfuns_double.pyx
+++ b/cmfrec/cfuns_double.pyx
@@ -1,3 +1,4 @@
+#cython: language_level=3
 import ctypes
 
 ctypedef double real_t
diff --git a/cmfrec/cfuns_double_plusblas.pyx b/cmfrec/cfuns_double_plusblas.pyx
index 9aed1d7..2fd61fe 100644
--- a/cmfrec/cfuns_double_plusblas.pyx
+++ b/cmfrec/cfuns_double_plusblas.pyx
@@ -1,3 +1,4 @@
+#cython: language_level=3
 import ctypes
 
 ctypedef double real_t
diff --git a/cmfrec/cfuns_float.pyx b/cmfrec/cfuns_float.pyx
index 1d04637..d9b5092 100644
--- a/cmfrec/cfuns_float.pyx
+++ b/cmfrec/cfuns_float.pyx
@@ -1,3 +1,4 @@
+#cython: language_level=3
 import ctypes
 
 ctypedef float real_t
diff --git a/cmfrec/cfuns_float_plusblas.pyx b/cmfrec/cfuns_float_plusblas.pyx
index 6953f03..e52283f 100644
--- a/cmfrec/cfuns_float_plusblas.pyx
+++ b/cmfrec/cfuns_float_plusblas.pyx
@@ -1,3 +1,4 @@
+#cython: language_level=3
 import ctypes
 
 ctypedef float real_t