diff --git a/AUTHORS b/AUTHORS
index 641e20441..7206125c8 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -71,3 +71,4 @@ Eric Hunsberger
 Simon Gomizelj
 Russel Winder
 Ben Webb
+Alexei Kozlenok
diff --git a/CHANGELOG b/CHANGELOG
index d1ac12a52..499660fa6 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,6 +1,10 @@
 2.8.5.dev0
 ----------
 
+- fix #1243: fixed injection to class breaking collection step, by replacing
+  generated interation with list traversal, added test to verify in python/collect.py
+  PR by Alexei Kozlenok, Thanks Ronny Pfannschmidt and Nicoddemus for the review and help.
+
 - fix #1074: precompute junitxml chunks instead of storing the whole tree in objects
   Thanks Bruno Oliveira for the report and Ronny Pfannschmidt for the PR
 
diff --git a/_pytest/python.py b/_pytest/python.py
index 4ff3f4fc0..926f503be 100644
--- a/_pytest/python.py
+++ b/_pytest/python.py
@@ -451,7 +451,7 @@ class PyCollector(PyobjMixin, pytest.Collector):
         seen = {}
         l = []
         for dic in dicts:
-            for name, obj in dic.items():
+            for name, obj in list(dic.items()):
                 if name in seen:
                     continue
                 seen[name] = True
diff --git a/testing/python/collect.py b/testing/python/collect.py
index 1a85faf9c..bebc13318 100644
--- a/testing/python/collect.py
+++ b/testing/python/collect.py
@@ -1076,3 +1076,26 @@ def test_dont_collect_non_function_callable(testdir):
         'WC2 *',
         '*1 passed, 1 pytest-warnings in *',
     ])
+
+
+def test_class_injection_does_not_break_collection(testdir):
+    """Tests whether injection during collection time will terminate testing.
+
+    In this case the error should not occur if the TestClass itself
+    is modified during collection time, and the original method list
+    is still used for collection.
+    """
+    testdir.makeconftest("""
+        from test_inject import TestClass
+        def pytest_generate_tests(metafunc):
+            TestClass.changed_var = {}
+    """)
+    testdir.makepyfile(test_inject='''
+         class TestClass(object):
+            def test_injection(self):
+                """Test being parametrized."""
+                pass
+    ''')
+    result = testdir.runpytest()
+    assert "RuntimeError: dictionary changed size during iteration" not in result.stdout.str()
+    result.stdout.fnmatch_lines(['*1 passed*'])