diff --git a/Eigen/src/Core/util/Constants.h b/Eigen/src/Core/util/Constants.h
index 37f18f2b4..c3dd3a09d 100644
--- a/Eigen/src/Core/util/Constants.h
+++ b/Eigen/src/Core/util/Constants.h
@@ -371,6 +371,10 @@ enum QRPreconditioners {
   FullPivHouseholderQRPreconditioner
 };
 
+#ifdef Success
+#error The preprocessor symbol 'Success' is defined, possibly by the X11 header file X.h
+#endif
+
 /** \ingroups enums
   * Enum for reporting the status of a computation. */
 enum ComputationInfo {