diff --git a/openquake/mbt/tools/mfd.py b/openquake/mbt/tools/mfd.py
index dbc7aa6e1..03e59b1ae 100644
--- a/openquake/mbt/tools/mfd.py
+++ b/openquake/mbt/tools/mfd.py
@@ -440,7 +440,7 @@ def stack(self, imfd):
                     print('Adding mag:', mag, occ)
 
                 tmp_mag = mags[-1] + bin_width
-                while tmp_mag < mag - 0.1 * `bin_width:
+                while tmp_mag < mag - 0.1 * bin_width:
                     tmp_mag += bin_width
                     delta += 1
                     if set([tmp_mag]) not in magset: