diff --git a/CHANGES.md b/CHANGES.md
index c29933fe5d9..1e51f3e9108 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -21,6 +21,8 @@
   and except clauses (#3423)
 - Fix a crash in preview advanced string processing where mixed implicitly concatenated
   regular and f-strings start with an empty span (#3463)
+- Fix a crash in preview advanced string processing where a standalone comment is placed
+  before a dict's value (#3469)
 - Do not put the closing quotes in a docstring on a separate line, even if the line is
   too long (#3430)
 - Long values in dict literals are now wrapped in parentheses; correspondingly
diff --git a/src/black/trans.py b/src/black/trans.py
index a5cf4955f13..0eb53e2b098 100644
--- a/src/black/trans.py
+++ b/src/black/trans.py
@@ -1866,7 +1866,7 @@ def _dict_or_lambda_match(LL: List[Leaf]) -> Optional[int]:
 
             for i, leaf in enumerate(LL):
                 # We MUST find a colon, it can either be dict's or lambda's colon...
-                if leaf.type == token.COLON:
+                if leaf.type == token.COLON and i < len(LL) - 1:
                     idx = i + 2 if is_empty_par(LL[i + 1]) else i + 1
 
                     # That colon MUST be followed by a string...
diff --git a/tests/data/preview/long_strings__regression.py b/tests/data/preview/long_strings__regression.py
index 5e8f012bc3e..ef9007f4ce1 100644
--- a/tests/data/preview/long_strings__regression.py
+++ b/tests/data/preview/long_strings__regression.py
@@ -543,6 +543,13 @@ async def foo(self):
     f'xxxxxxxxxxxxx xxxx xx xxxxxxxxxx. xxxxx: {x.xxx}',
 )
 
+# Regression test for https://github.com/psf/black/issues/3455.
+a_dict = {
+    "/this/is/a/very/very/very/very/very/very/very/very/very/very/long/key/without/spaces":
+        # And there is a comment before the value
+        ("item1", "item2", "item3"),
+}
+
 
 # output
 
@@ -1221,3 +1228,10 @@ async def foo(self):
         f"xxxxxxxxxxxxx xxxx xx xxxxxxxxxx. xxxxx: {x.xxx}"
     ),
 )
+
+# Regression test for https://github.com/psf/black/issues/3455.
+a_dict = {
+    "/this/is/a/very/very/very/very/very/very/very/very/very/very/long/key/without/spaces":
+    # And there is a comment before the value
+    ("item1", "item2", "item3"),
+}