diff --git a/include/nlohmann/detail/input/binary_reader.hpp b/include/nlohmann/detail/input/binary_reader.hpp
index b8830f2174..43a163fa00 100644
--- a/include/nlohmann/detail/input/binary_reader.hpp
+++ b/include/nlohmann/detail/input/binary_reader.hpp
@@ -1952,7 +1952,7 @@ class binary_reader
             return false;
         }
 
-        if (size_and_type.first != string_t::npos)
+        if (size_and_type.first != npos)
         {
             if (size_and_type.second != 0)
             {
@@ -2185,7 +2185,7 @@ class binary_reader
                     for (auto i : dim)
                     {
                         result *= i;
-                        if (result == 0 || result == string_t::npos) // because dim elements shall not have zeros, result = 0 means overflow happened; it also can't be string_t::npos as it is used to initialize size in get_ubjson_size_type()
+                        if (result == 0 || result == npos) // because dim elements shall not have zeros, result = 0 means overflow happened; it also can't be npos as it is used to initialize size in get_ubjson_size_type()
                         {
                             return sax->parse_error(chars_read, get_token_string(), out_of_range::create(408, exception_message(input_format, "excessive ndarray size caused overflow", "size"), nullptr));
                         }
@@ -2231,7 +2231,7 @@ class binary_reader
     */
     bool get_ubjson_size_type(std::pair<std::size_t, char_int_type>& result, bool inside_ndarray = false)
     {
-        result.first = string_t::npos; // size
+        result.first = npos; // size
         result.second = 0; // type
         bool is_ndarray = false;
 
@@ -2493,7 +2493,7 @@ class binary_reader
         // if bit-8 of size_and_type.second is set to 1, encode bjdata ndarray as an object in JData annotated array format (https://github.com/NeuroJSON/jdata):
         // {"_ArrayType_" : "typeid", "_ArraySize_" : [n1, n2, ...], "_ArrayData_" : [v1, v2, ...]}
 
-        if (input_format == input_format_t::bjdata && size_and_type.first != string_t::npos && (size_and_type.second & (1 << 8)) != 0)
+        if (input_format == input_format_t::bjdata && size_and_type.first != npos && (size_and_type.second & (1 << 8)) != 0)
         {
             size_and_type.second &= ~(static_cast<char_int_type>(1) << 8);  // use bit 8 to indicate ndarray, here we remove the bit to restore the type marker
             auto it = std::lower_bound(bjd_types_map.begin(), bjd_types_map.end(), size_and_type.second, [](const bjd_type & p, char_int_type t)
@@ -2536,7 +2536,7 @@ class binary_reader
             return (sax->end_array() && sax->end_object());
         }
 
-        if (size_and_type.first != string_t::npos)
+        if (size_and_type.first != npos)
         {
             if (JSON_HEDLEY_UNLIKELY(!sax->start_array(size_and_type.first)))
             {
@@ -2599,7 +2599,7 @@ class binary_reader
         }
 
         // do not accept ND-array size in objects in BJData
-        if (input_format == input_format_t::bjdata && size_and_type.first != string_t::npos && (size_and_type.second & (1 << 8)) != 0)
+        if (input_format == input_format_t::bjdata && size_and_type.first != npos && (size_and_type.second & (1 << 8)) != 0)
         {
             auto last_token = get_token_string();
             return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
@@ -2607,7 +2607,7 @@ class binary_reader
         }
 
         string_t key;
-        if (size_and_type.first != string_t::npos)
+        if (size_and_type.first != npos)
         {
             if (JSON_HEDLEY_UNLIKELY(!sax->start_object(size_and_type.first)))
             {
@@ -2951,6 +2951,8 @@ class binary_reader
     }
 
   private:
+    static JSON_INLINE_VARIABLE constexpr std::size_t npos = static_cast<std::size_t>(-1);
+
     /// input adapter
     InputAdapterType ia;
 
@@ -3002,5 +3004,10 @@ class binary_reader
 #undef JSON_BINARY_READER_MAKE_BJD_TYPES_MAP_
 };
 
+#ifndef JSON_HAS_CPP_17
+    template<typename BasicJsonType, typename InputAdapterType, typename SAX>
+    constexpr std::size_t binary_reader<BasicJsonType, InputAdapterType, SAX>::npos;
+#endif
+
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 97e0c12f28..b111c68c60 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -10947,7 +10947,7 @@ class binary_reader
             return false;
         }
 
-        if (size_and_type.first != string_t::npos)
+        if (size_and_type.first != npos)
         {
             if (size_and_type.second != 0)
             {
@@ -11180,7 +11180,7 @@ class binary_reader
                     for (auto i : dim)
                     {
                         result *= i;
-                        if (result == 0 || result == string_t::npos) // because dim elements shall not have zeros, result = 0 means overflow happened; it also can't be string_t::npos as it is used to initialize size in get_ubjson_size_type()
+                        if (result == 0 || result == npos) // because dim elements shall not have zeros, result = 0 means overflow happened; it also can't be npos as it is used to initialize size in get_ubjson_size_type()
                         {
                             return sax->parse_error(chars_read, get_token_string(), out_of_range::create(408, exception_message(input_format, "excessive ndarray size caused overflow", "size"), nullptr));
                         }
@@ -11226,7 +11226,7 @@ class binary_reader
     */
     bool get_ubjson_size_type(std::pair<std::size_t, char_int_type>& result, bool inside_ndarray = false)
     {
-        result.first = string_t::npos; // size
+        result.first = npos; // size
         result.second = 0; // type
         bool is_ndarray = false;
 
@@ -11488,7 +11488,7 @@ class binary_reader
         // if bit-8 of size_and_type.second is set to 1, encode bjdata ndarray as an object in JData annotated array format (https://github.com/NeuroJSON/jdata):
         // {"_ArrayType_" : "typeid", "_ArraySize_" : [n1, n2, ...], "_ArrayData_" : [v1, v2, ...]}
 
-        if (input_format == input_format_t::bjdata && size_and_type.first != string_t::npos && (size_and_type.second & (1 << 8)) != 0)
+        if (input_format == input_format_t::bjdata && size_and_type.first != npos && (size_and_type.second & (1 << 8)) != 0)
         {
             size_and_type.second &= ~(static_cast<char_int_type>(1) << 8);  // use bit 8 to indicate ndarray, here we remove the bit to restore the type marker
             auto it = std::lower_bound(bjd_types_map.begin(), bjd_types_map.end(), size_and_type.second, [](const bjd_type & p, char_int_type t)
@@ -11531,7 +11531,7 @@ class binary_reader
             return (sax->end_array() && sax->end_object());
         }
 
-        if (size_and_type.first != string_t::npos)
+        if (size_and_type.first != npos)
         {
             if (JSON_HEDLEY_UNLIKELY(!sax->start_array(size_and_type.first)))
             {
@@ -11594,7 +11594,7 @@ class binary_reader
         }
 
         // do not accept ND-array size in objects in BJData
-        if (input_format == input_format_t::bjdata && size_and_type.first != string_t::npos && (size_and_type.second & (1 << 8)) != 0)
+        if (input_format == input_format_t::bjdata && size_and_type.first != npos && (size_and_type.second & (1 << 8)) != 0)
         {
             auto last_token = get_token_string();
             return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
@@ -11602,7 +11602,7 @@ class binary_reader
         }
 
         string_t key;
-        if (size_and_type.first != string_t::npos)
+        if (size_and_type.first != npos)
         {
             if (JSON_HEDLEY_UNLIKELY(!sax->start_object(size_and_type.first)))
             {
@@ -11946,6 +11946,8 @@ class binary_reader
     }
 
   private:
+    static JSON_INLINE_VARIABLE constexpr std::size_t npos = static_cast<std::size_t>(-1);
+
     /// input adapter
     InputAdapterType ia;
 
@@ -11997,6 +11999,11 @@ class binary_reader
 #undef JSON_BINARY_READER_MAKE_BJD_TYPES_MAP_
 };
 
+#ifndef JSON_HAS_CPP_17
+    template<typename BasicJsonType, typename InputAdapterType, typename SAX>
+    constexpr std::size_t binary_reader<BasicJsonType, InputAdapterType, SAX>::npos;
+#endif
+
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END