diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index fa3815216..d66b0819f 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -29,7 +29,7 @@ public:
     ATermMap(const ATermMap & map);
     ~ATermMap();
 
-    ATermMap & ATermMap::operator = (const ATermMap & map);
+    ATermMap & operator = (const ATermMap & map);
         
     void set(ATerm key, ATerm value);
     void set(const string & key, ATerm value);