diff --git a/mypy/messages.py b/mypy/messages.py
index c6378c264757..4c79f94ad5b8 100644
--- a/mypy/messages.py
+++ b/mypy/messages.py
@@ -21,6 +21,7 @@
 
 import mypy.typeops
 from mypy import errorcodes as codes, message_registry
+from mypy.constant_fold import constant_fold_expr
 from mypy.erasetype import erase_type
 from mypy.errorcodes import ErrorCode
 from mypy.errors import (
@@ -50,7 +51,6 @@
     MypyFile,
     NameExpr,
     ReturnStmt,
-    StrExpr,
     SymbolNode,
     SymbolTable,
     TypeInfo,
@@ -808,10 +808,10 @@ def incompatible_argument(
                     )
                     arg_label = f'"{arg_name}"'
                 if isinstance(outer_context, IndexExpr) and isinstance(
-                    outer_context.index, StrExpr
+                    index := constant_fold_expr(outer_context.index, "<unused>"), str
                 ):
                     msg = 'Value of "{}" has incompatible type {}; expected {}'.format(
-                        outer_context.index.value,
+                        index,
                         quote_type_string(arg_type_str),
                         quote_type_string(expected_type_str),
                     )