diff --git a/src/yajson/yajson.h b/src/yajson/yajson.h
index 5571fd9..c8560ea 100644
--- a/src/yajson/yajson.h
+++ b/src/yajson/yajson.h
@@ -382,7 +382,7 @@ inline std::string Value::string() const {
     return isNull() ? std::string() : _instance->string();
 }
 
-bool Value::isNull() const {
+inline bool Value::isNull() const {
     return nullptr == _instance;
 }
 
@@ -1320,4 +1320,4 @@ inline void Object::set(const std::string& key, const Value& value) {
 }
 
 #undef YaJsonAssert
-}
\ No newline at end of file
+}