diff --git a/src/tools/Random.cpp b/src/tools/Random.cpp
index 530bae72c7c6d1b3640899db73d8a8665cfcbc25..7ed31d5d574e0e865e441436bd8194169f7fb7c9 100644
--- a/src/tools/Random.cpp
+++ b/src/tools/Random.cpp
@@ -51,6 +51,7 @@ Random::Random(const std::string & name):
 }
 
 void Random::setSeed(int idum_) {
+  if(idum_>0) idum_=-idum_;
   idum=idum_;
   incPrec=false;
 }