diff --git a/fluents/dataset.py b/fluents/dataset.py
index b7fde34..de3f566 100644
--- a/fluents/dataset.py
+++ b/fluents/dataset.py
@@ -412,6 +412,9 @@ def write_ftsv(fd, ds):
 
     # Write data
     m = ds.asarray()
+    if type == 'category':
+        m = m.astype('i')
+
     y, x = m.shape
     for j in range(y):
         for i in range(x):