diff --git a/src/ifcopenshell-python/ifcopenshell/express/rule_compiler.py b/src/ifcopenshell-python/ifcopenshell/express/rule_compiler.py
index fc61ce899b..1ed05df3d0 100644
--- a/src/ifcopenshell-python/ifcopenshell/express/rule_compiler.py
+++ b/src/ifcopenshell-python/ifcopenshell/express/rule_compiler.py
@@ -335,17 +335,18 @@ def process_domain_rule(context):
 assert ({context.expression}) is not False
 """
 
+def wrap_parens(s):
+    s = str(s)
+    if " " in s:
+        s = '(%s)' % s
+    return s
+
 def process_expression(context):
-    def wrap(s):
-        s = str(s)
-        if " " in s:
-            s = '(%s)' % s
-        return s
 
     def concat(a, b, **kwargs):
         return " ".join(map(str, sum(zip(
             [None] + a.branches(**kwargs),
-            map(wrap, b.branches(**kwargs))
+            map(wrap_parens, b.branches(**kwargs))
         ), ())[1:]))
 
     if context.rel_op_extended:
@@ -443,11 +444,10 @@ def simple_concat(context):
     # correction: still necessary, apparently.
     # branches = list(map(str, context.branches()))
 
-    concat = ""
     if len(branches) == 2 and branches[0] == 'not':
-        concat = " "
-
-    v = concat.join(branches)
+        return f"{branches[0]} {wrap_parens(branches[1])}"
+    else:
+        v = "".join(branches)
 
     return v