diff --git a/master.cfg b/master.cfg
index bd8062e..fd17c31 100644
--- a/master.cfg
+++ b/master.cfg
@@ -1291,8 +1291,8 @@ class RunTestGDBSolaris_Common (BuildAndTestGDBFactory):
 class RunTestGDBPlainSolaris_c64 (RunTestGDBSolaris_Common):
     """Compiling for Solaris"""
     def __init__ (self, **kwargs):
-        self.extra_CFLAGS.append ('-m64')
-        self.extra_CXXFLAGS.append ('-m64')
+        self.extra_CFLAGS += [ '-m64' ]
+        self.extra_CXXFLAGS += [ '-m64' ]
         BuildAndTestGDBFactory.__init__ (self, **kwargs)
 
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