diff --git a/CHANGES b/CHANGES
index 8796c861..05e026f8 100644
--- a/CHANGES
+++ b/CHANGES
@@ -4,6 +4,9 @@ Change Log
 
 Version 3.0.7 -
 ---------------
+- Fixed bug #345, in which delimitedList changed expressions in place
+  using expr.streamline(). Reported by Kim Gräsman, thanks!
+
 - Added optional "min" and "max" arguments to `delimited_list`. PR
   submitted by Marius, thanks!
 
diff --git a/pyparsing/__init__.py b/pyparsing/__init__.py
index cd4d41f3..7b5bb1df 100644
--- a/pyparsing/__init__.py
+++ b/pyparsing/__init__.py
@@ -126,7 +126,7 @@ def __repr__(self):
 
 
 __version_info__ = version_info(3, 0, 7, "final", 0)
-__version_time__ = "04 Dec 2021 08:48 UTC"
+__version_time__ = "15 Dec 2021 05:56 UTC"
 __version__ = __version_info__.__version__
 __versionTime__ = __version_time__
 __author__ = "Paul McGuire <ptmcg.gm+pyparsing@gmail.com>"
diff --git a/pyparsing/helpers.py b/pyparsing/helpers.py
index 8d61301a..19dd0e04 100644
--- a/pyparsing/helpers.py
+++ b/pyparsing/helpers.py
@@ -40,7 +40,7 @@ def delimited_list(
         expr = ParserElement._literalStringClass(expr)
 
     dlName = "{expr} [{delim} {expr}]...{end}".format(
-        expr=str(expr.streamline()),
+        expr=str(expr.copy().streamline()),
         delim=str(delim),
         end=" [{}]".format(str(delim)) if allow_trailing_delim else "",
     )
diff --git a/tests/test_unit.py b/tests/test_unit.py
index 95183a3a..0d957f1b 100644
--- a/tests/test_unit.py
+++ b/tests/test_unit.py
@@ -6721,6 +6721,43 @@ def add_total(tokens):
             "noop parse action changed ParseResults structure",
         )
 
+    def testParseActionWithDelimitedList(self):
+        class AnnotatedToken(object):
+            def __init__(self, kind, elements):
+                self.kind = kind
+                self.elements = elements
+
+            def __str__(self):
+                return 'AnnotatedToken(%r, %r)' % (self.kind, self.elements)
+
+            def __eq__(self, other):
+                return type(self) == type(other) and self.kind == other.kind and self.elements == other.elements
+
+            __repr__ = __str__
+
+        def annotate(name):
+            def _(t):
+                return AnnotatedToken(name, t.asList())
+            return _
+
+        identifier = pp.Word(pp.srange('[a-z0-9]'))
+        numeral = pp.Word(pp.nums)
+
+        named_number_value = pp.Suppress('(') + numeral + pp.Suppress(')')
+        named_number = identifier + named_number_value
+
+        named_number_list = (pp.Suppress('{') +
+                             pp.Group(pp.Optional(pp.delimitedList(named_number))) +
+                             pp.Suppress('}'))
+
+        # repro but in #345 - delimitedList silently changes contents of named_number
+        named_number_value.setParseAction(annotate("val"))
+
+        test_string = "{ x1(1), x2(2) }"
+        expected = [['x1', AnnotatedToken("val", ['1']), 'x2', AnnotatedToken("val", ['2'])]]
+
+        self.assertParseAndCheckList(named_number_list, test_string, expected)
+
     def testParseResultsNameBelowUngroupedName(self):
 
         rule_num = pp.Regex("[0-9]+")("LIT_NUM*")