diff --git a/source/CHANGELOG.md b/source/CHANGELOG.md
index 27ad82fbb..37d190ff9 100644
--- a/source/CHANGELOG.md
+++ b/source/CHANGELOG.md
@@ -1,3 +1,9 @@
+# Unreleased
+
+* C++: fixing a regression introduced in 2.9.6.2
+  - `std::to_string` is not compatible to `--ansi` standard
+    [[#534](https://github.com/BNFC/bnfc/issues/534)]
+
 # 2.9.6.2
 
 Andreas Abel <andreas.abel@gu.se>,  January 2026
diff --git a/source/src/BNFC/Backend/C/CFtoBisonC.hs b/source/src/BNFC/Backend/C/CFtoBisonC.hs
index 3fd074da7..8c9692f22 100644
--- a/source/src/BNFC/Backend/C/CFtoBisonC.hs
+++ b/source/src/BNFC/Backend/C/CFtoBisonC.hs
@@ -119,6 +119,10 @@ header mode cf = unlines $ concat
   , when (stlParser mode)
     [ "#include <algorithm> /* for std::reverse */"  -- mandatory e.g. with GNU C++ 11
     , "#include \"" ++ ("ParserError" <.> h) ++ "\""  -- for throwing parser_error in C++
+
+    -- Commelina, 2026-03-05, issue #534: @std::to_string@ is invalid with @--ansi@
+    -- TODO: Choose to use @std::to_string@ or not according to the @--ansi@ flag
+    , "#include <sstream>"
     ]
   , [ "#include <stdio.h>"
     , "#include <stdlib.h>"
@@ -186,9 +190,15 @@ errorHandler mode = case mode of
       , "{"
       , "  std::string error_msg = msg;"
       , "  if (loc) {"
-      , "    error_msg += \" at line \" + std::to_string(loc->first_line) +"
-      , "                 \", column \" + std::to_string(loc->first_column);"
+
+      -- Commelina, 2026-03-05, issue #534: @std::to_string@ is invalid with @--ansi@
+      -- TODO: Choose to use @std::to_string@ or not according to the @--ansi@ flag
+      , "    std::ostringstream oss;"
+      , "    oss << \" at line \" << loc->first_line"
+      , "        << \", column \" << loc->first_column;"
+      , "    error_msg += oss.str();"
       , "  }"
+
       , "  if (scanner) {"
       , "    error_msg += \": '\" + std::string(" ++ name ++ "get_text(scanner)) + \"'\";"
       , "  }"
diff --git a/testing/src/ParameterizedTests.hs b/testing/src/ParameterizedTests.hs
index 3d10f4a56..91b556de6 100644
--- a/testing/src/ParameterizedTests.hs
+++ b/testing/src/ParameterizedTests.hs
@@ -390,16 +390,11 @@ parameters = concat
     -- Functor (Haskell & Agda)
   , [ haskellAgdaFunctorParameters]
     -- C++ (extras)
-
-  -- Note: Disabled on 2026-03-04. Re-enable it after
-  --       https://github.com/BNFC/bnfc/issues/534 is resolved.
-  --
-  -- , [ cBase { tpName = "C++ (with line numbers)"
-  --           , tpBnfcOptions = ["--cpp", "-l"] }
-  --   , cBase { tpName = "C++ (with namespace)"
-  --           , tpBnfcOptions = ["--cpp", "-p foobar"] }
-  --   ]
-
+  , [ cBase { tpName = "C++ (with line numbers)"
+            , tpBnfcOptions = ["--cpp", "-l"] }
+    , cBase { tpName = "C++ (with namespace)"
+            , tpBnfcOptions = ["--cpp", "-p foobar"] }
+    ]
     -- C
   , [ TP { tpName = "C"
          , tpBnfcOptions = ["--c"]
@@ -433,12 +428,8 @@ parameters = concat
     -- C++ (basic)
   , [ cBase { tpName = "C++ (no STL)"
             , tpBnfcOptions = ["--cpp-nostl"] }
-
-    -- Note: Disabled on 2026-03-04. Re-enable it after
-    --       https://github.com/BNFC/bnfc/issues/534 is resolved.
-    --
-    -- , cBase { tpName = "C++"
-    --         , tpBnfcOptions = ["--cpp"] }
+    , cBase { tpName = "C++"
+            , tpBnfcOptions = ["--cpp"] }
     ]
     -- Agda
   , [ haskellAgdaParameters ]