diff --git a/include/nlohmann/detail/output/serializer.hpp b/include/nlohmann/detail/output/serializer.hpp
index 20a65d76e..80ee84c8c 100644
--- a/include/nlohmann/detail/output/serializer.hpp
+++ b/include/nlohmann/detail/output/serializer.hpp
@@ -844,8 +844,15 @@ class serializer
             if (state == UTF8_ACCEPT)
             {
                 const auto* const data = reinterpret_cast<const unsigned char*>(s.data());
+                // A run can only be non-empty when the very first byte is one
+                // the scanner may copy, so test that single byte before paying
+                // for the scan. Without it, text whose characters all have to be
+                // escaped - CJK under ensure_ascii, where every byte is >= 0x80 -
+                // runs the scanner once per character only to be told zero.
                 const std::size_t run = EnsureAscii
-                                        ? find_ascii_copyable_run(data + i, s.size() - i)
+                                        ? (is_ascii_copyable(data[i])
+                                           ? find_ascii_copyable_run(data + i, s.size() - i)
+                                           : 0)
                                         : string_bulk_run(data + i, s.size() - i);
                 if (run != 0)
                 {
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 04438d1f7..26d0943da 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -21904,8 +21904,15 @@ class serializer
             if (state == UTF8_ACCEPT)
             {
                 const auto* const data = reinterpret_cast<const unsigned char*>(s.data());
+                // A run can only be non-empty when the very first byte is one
+                // the scanner may copy, so test that single byte before paying
+                // for the scan. Without it, text whose characters all have to be
+                // escaped - CJK under ensure_ascii, where every byte is >= 0x80 -
+                // runs the scanner once per character only to be told zero.
                 const std::size_t run = EnsureAscii
-                                        ? find_ascii_copyable_run(data + i, s.size() - i)
+                                        ? (is_ascii_copyable(data[i])
+                                           ? find_ascii_copyable_run(data + i, s.size() - i)
+                                           : 0)
                                         : string_bulk_run(data + i, s.size() - i);
                 if (run != 0)
                 {