diff --git a/frappy/datatypes.py b/frappy/datatypes.py
index 4da0b87..4266939 100644
--- a/frappy/datatypes.py
+++ b/frappy/datatypes.py
@@ -1035,7 +1035,7 @@ class StructOf(DataType):
         return self(dict(value))
 
     def format_value(self, value, unit=None):
-        return '{%s}' % (', '.join(['%s=%s' % (k, self.members[k].format_value(v)) for k, v in sorted(value.items())]))
+        return '{%s}' % (', '.join(['%s=%s' % (k, self.members[k].format_value(v)) for k, v in value.items()]))
 
     def compatible(self, other):
         try:
diff --git a/test/test_datatypes.py b/test/test_datatypes.py
index 2cf98bc..96cfa95 100644
--- a/test/test_datatypes.py
+++ b/test/test_datatypes.py
@@ -474,7 +474,7 @@ def test_StructOf():
     assert dt.import_value({'an_int': 13, 'a_string': 'WFEC'}) == {
         'a_string': 'WFEC', 'an_int': 13}
 
-    assert dt.format_value({'an_int':2, 'a_string':'Z'}) == "{a_string='Z', an_int=2}"
+    assert dt.format_value({'an_int': 2, 'a_string': 'Z'}) == "{an_int=2, a_string='Z'}"
 
     dt = StructOf(['optionalmember'], optionalmember=EnumType('myenum', single=0))
     copytest(dt)