diff --git a/CHANGES b/CHANGES
index 42ac09da0ed..3ef0bb53eaf 100644
--- a/CHANGES
+++ b/CHANGES
@@ -68,6 +68,8 @@ Features added
 Bugs fixed
 ----------
 
+* #9210: viewcode: crashed if non importable modules found on parallel build
+
 Testing
 --------
 
diff --git a/sphinx/ext/viewcode.py b/sphinx/ext/viewcode.py
index f2a42f33f82..5728f607761 100644
--- a/sphinx/ext/viewcode.py
+++ b/sphinx/ext/viewcode.py
@@ -150,10 +150,11 @@ def env_merge_info(app: Sphinx, env: BuildEnvironment, docnames: Iterable[str],
         if modname not in env._viewcode_modules:  # type: ignore
             env._viewcode_modules[modname] = entry  # type: ignore
         else:
-            used = env._viewcode_modules[modname][2]  # type: ignore
-            for fullname, docname in entry[2].items():
-                if fullname not in used:
-                    used[fullname] = docname
+            if env._viewcode_modules[modname]:  # type: ignore
+                used = env._viewcode_modules[modname][2]  # type: ignore
+                for fullname, docname in entry[2].items():
+                    if fullname not in used:
+                        used[fullname] = docname
 
 
 def env_purge_doc(app: Sphinx, env: BuildEnvironment, docname: str) -> None:
diff --git a/tests/test_util_inspect.py b/tests/test_util_inspect.py
index 95e36ba14b6..dd9bd14a3dd 100644
--- a/tests/test_util_inspect.py
+++ b/tests/test_util_inspect.py
@@ -214,7 +214,11 @@ def test_signature_annotations():
 
     # optional union
     sig = inspect.signature(f20)
-    assert stringify_signature(sig) == '() -> Optional[Union[int, str]]'
+    if sys.version_info < (3, 7):
+        assert stringify_signature(sig) in ('() -> Optional[Union[int, str]]',
+                                            '() -> Optional[Union[str, int]]')
+    else:
+        assert stringify_signature(sig) == '() -> Optional[Union[int, str]]'
 
     # Any
     sig = inspect.signature(f14)