diff --git a/bdpy/mri/fmriprep.py b/bdpy/mri/fmriprep.py
index 6ebff3a2..e0ed2f40 100644
--- a/bdpy/mri/fmriprep.py
+++ b/bdpy/mri/fmriprep.py
@@ -385,7 +385,7 @@ def data(self):
 
     @property
     def xyz(self):
-        if self.__dtype is 'surface':
+        if self.__dtype == 'surface':
             raise NotImplementedError('Vertex xyz coordinates are not implemented yet.')
         return self.__xyz
 
@@ -395,7 +395,7 @@ def index(self):
 
     @property
     def n_vertex(self):
-        if self.__dtype is not 'surface':
+        if self.__dtype != 'surface':
             raise TypeError('Not surface data.')
         return self.__n_vertex