diff --git a/coverage/results.py b/coverage/results.py
index dd14c8a27..39612a8a7 100644
--- a/coverage/results.py
+++ b/coverage/results.py
@@ -202,8 +202,7 @@ def missing_branch_arcs(self) -> dict[TLineNo, list[TLineNo]]:
         branch_lines = set(self._branch_lines())
         mba = collections.defaultdict(list)
         for l1, l2 in missing:
-            if l1 == l2:
-                continue
+            assert l1 != l2, f"In {self.filename}, didn't expect {l1} == {l2}"
             if l1 in branch_lines:
                 mba[l1].append(l2)
         return mba
diff --git a/tests/test_json.py b/tests/test_json.py
index 0ccfd84c0..e5b116ac4 100644
--- a/tests/test_json.py
+++ b/tests/test_json.py
@@ -559,3 +559,20 @@ def test_context_non_relative(self) -> None:
 
     def test_context_relative(self) -> None:
         self.run_context_test(relative_files=True)
+
+    def test_l1_equals_l2(self) -> None:
+        # In results.py, we had a line checking `if l1 == l2` that was never
+        # true.  This test makes it true. The annotations are essential, I
+        # don't know why.
+        self.make_file("wtf.py", """\
+            def function(
+                x: int,
+                y: int,
+            ) -> None:
+                return x + y
+
+            assert function(3, 5) == 8
+            """)
+        cov = coverage.Coverage(branch=True)
+        mod = self.start_import_stop(cov, "wtf")
+        cov.json_report(mod)