diff --git a/ixwebsocket/IXSocketOpenSSL.h b/ixwebsocket/IXSocketOpenSSL.h
index 2edab63e..6070f8ae 100644
--- a/ixwebsocket/IXSocketOpenSSL.h
+++ b/ixwebsocket/IXSocketOpenSSL.h
@@ -50,10 +50,10 @@ namespace ix
         bool openSSLServerHandshake(std::string& errMsg);
 
         // Required for OpenSSL < 1.1
-        void openSSLLockingCallback(int mode,
-                                    int type,
-                                    const char* /*file*/,
-                                    int /*line*/);
+        static void openSSLLockingCallback(int mode,
+                                           int type,
+                                           const char* /*file*/,
+                                           int /*line*/);
 
         SSL* _ssl_connection;
         SSL_CTX* _ssl_context;