diff --git a/trezorlib/tests/unit_tests/test_protobuf.py b/trezorlib/tests/unit_tests/test_protobuf.py
index d4299e6e2..0ea04ea09 100644
--- a/trezorlib/tests/unit_tests/test_protobuf.py
+++ b/trezorlib/tests/unit_tests/test_protobuf.py
@@ -22,13 +22,15 @@ from trezorlib import protobuf
 
 
 class PrimitiveMessage(protobuf.MessageType):
-    FIELDS = {
-        0: ("uvarint", protobuf.UVarintType, 0),
-        1: ("svarint", protobuf.SVarintType, 0),
-        2: ("bool", protobuf.BoolType, 0),
-        3: ("bytes", protobuf.BytesType, 0),
-        4: ("unicode", protobuf.UnicodeType, 0),
-    }
+    @classmethod
+    def get_fields(cls):
+        return {
+            0: ("uvarint", protobuf.UVarintType, 0),
+            1: ("svarint", protobuf.SVarintType, 0),
+            2: ("bool", protobuf.BoolType, 0),
+            3: ("bytes", protobuf.BytesType, 0),
+            4: ("unicode", protobuf.UnicodeType, 0),
+        }
 
 
 def load_uvarint(buffer):