diff --git a/CHANGES.rst b/CHANGES.rst
index 34bb8e139..81e4fbb19 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -23,8 +23,10 @@ upgrading your version of coverage.py.
 Unreleased
 ----------
 
-Nothing yet.
+- Fix: in some cases, coverage could fail with a RuntimeError: "Set changed
+  size during iteration." This is now fixed, closing `issue 1733`_.
 
+.. _issue 1733: https://github.com/nedbat/coveragepy/issues/1733
 
 .. scriv-start-here
 
diff --git a/coverage/collector.py b/coverage/collector.py
index 87cd620e3..dcb8a30dd 100644
--- a/coverage/collector.py
+++ b/coverage/collector.py
@@ -513,9 +513,12 @@ def flush_data(self) -> bool:
                 # these packed ints.
                 arc_data: Dict[str, List[TArc]] = {}
                 packed_data = cast(Dict[str, Set[int]], self.data)
-                for fname, packeds in packed_data.items():
+
+                # The list() here and in the inner loop are to get a clean copy
+                # even as tracers are continuing to add data.
+                for fname, packeds in list(packed_data.items()):
                     tuples = []
-                    for packed in packeds:
+                    for packed in list(packeds):
                         l1 = packed & 0xFFFFF
                         l2 = (packed & (0xFFFFF << 20)) >> 20
                         if packed & (1 << 40):