diff --git a/src/google/protobuf/compiler/cpp/cpp_map_field.cc b/src/google/protobuf/compiler/cpp/cpp_map_field.cc
index 44c913868a56..b1e3c1e50cbc 100644
--- a/src/google/protobuf/compiler/cpp/cpp_map_field.cc
+++ b/src/google/protobuf/compiler/cpp/cpp_map_field.cc
@@ -229,7 +229,10 @@ void MapFieldGenerator::GenerateSerializeWithCachedSizesToArray(
   if (utf8_check) {
     format(
         "struct Utf8Check {\n"
-        "  static void Check(ConstPtr p) {\n");
+        "  static void Check(ConstPtr p) {\n"
+        // p may be unused when GetUtf8CheckMode evaluates to kNone,
+        // thus disabling the validation.
+        "    (void)p;\n");
     format.Indent();
     format.Indent();
     if (string_key) {