diff --git a/src/ctypesgen/expressions.py b/src/ctypesgen/expressions.py
index 0e5f77d..251c7e0 100644
--- a/src/ctypesgen/expressions.py
+++ b/src/ctypesgen/expressions.py
@@ -230,7 +230,7 @@ def __init__(self, op, format, base, attribute):
         # Fortunately, the processor module does the same thing to
         # the struct member name.
         if self.attribute in keyword.kwlist:
-            self.attribute = "_" + self.attribute
+            self.attribute += "_"
 
     def visit(self, visitor):
         self.base.visit(visitor)