diff --git a/tests/test_callfsl.py b/tests/test_callfsl.py
index e137160e46e341d0d9744aaafa3b8bdfe8e3885a..85176b2124e565a0ad13df3131ce75cd535eb55f 100644
--- a/tests/test_callfsl.py
+++ b/tests/test_callfsl.py
@@ -31,10 +31,10 @@ def setup_module():
 # or 'fslinfo ...'
 def mock_check_output(args):
     if args[0].endswith('fslinfo'):
-        return 'info'
+        return 'info'.encode('utf-8')
 
     img = nib.load(args[-2])
-    return str(img.get_data().mean())
+    return str(img.get_data().mean()).encode('utf-8')
 
 
 def test_callfsl():