diff --git a/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h b/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h
index 8d23cb4..a25c9cd 100644
--- a/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h
+++ b/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h
@@ -130,6 +130,8 @@
     }
 private:
     Scalar epsfcn;
+
+    NumericalDiff& operator=(const NumericalDiff&);
 };
 
 //vim: ai ts=4 sts=4 et sw=4