diff --git a/json/except.cpp b/json/except.cpp
index 430e597e..6019ffd3 100644
--- a/json/except.cpp
+++ b/json/except.cpp
@@ -26,3 +26,9 @@ json::parse_error::parse_error (const std::string &_what, size_t _line):
     error (_what),
     line  (_line)
 { ; }
+
+
+//-----------------------------------------------------------------------------
+json::key_error::key_error (std::string _what):
+    error (std::move (_what))
+{ ; }
diff --git a/json/except.hpp b/json/except.hpp
index ec9be0b9..8d7ffb5b 100644
--- a/json/except.hpp
+++ b/json/except.hpp
@@ -31,11 +31,13 @@ namespace json {
         using runtime_error::runtime_error;
     };
 
+
     /// Base class for all type conversion errors
     struct type_error : public error {
         using error::error;
     };
 
+
     /// Base class for errors thrown during parsing
     struct parse_error : public error {
         using error::error;
@@ -45,10 +47,17 @@ namespace json {
         size_t line;
     };
 
+
     /// Base class for errors thrown during schema validation
     struct schema_error : public error {
         using error::error;
     };
+
+
+    /// Exception class for invalid object indexing
+    struct key_error : public error {
+        key_error (std::string);
+    };
 }
 
 #endif
diff --git a/json/tree.cpp b/json/tree.cpp
index 0f6a601e..359728de 100644
--- a/json/tree.cpp
+++ b/json/tree.cpp
@@ -319,15 +319,25 @@ void
 json::tree::object::insert (const std::string &_key, unique_ptr<json::tree::node> &&value)
         { m_values[_key] = move(value); }
 
-
-const json::tree::node&
-json::tree::object::operator[](const std::string &key) const {
+//-----------------------------------------------------------------------------
+json::tree::node&
+json::tree::object::operator[](const std::string &key)
+{
     auto value = m_values.find (key);
-    if (value == m_values.end ()) {
-        ostringstream ss;
-        ss << "no key: " << key;
-        throw json::error (ss.str());
-    }
+    if (value == m_values.end ())
+        throw json::key_error (key);
+
+    return *value->second;
+}
+
+
+//-----------------------------------------------------------------------------
+const json::tree::node&
+json::tree::object::operator[](const std::string &key) const
+{
+    auto value = m_values.find (key);
+    if (value == m_values.end ())
+        throw json::key_error (key);
 
     return *value->second;
 }