diff --git a/src/FunctionCombine.cpp b/src/FunctionCombine.cpp
index 48e3a5dd782dfb9c3108b2429a87ca567bc62b3c..920f37ca15dc7fc33245f8a09b964aa5d5cc8b3a 100644
--- a/src/FunctionCombine.cpp
+++ b/src/FunctionCombine.cpp
@@ -71,10 +71,10 @@ powers(getNumberOfArguments(),1.0)
   if(period.size()==0){
   }else if(period.size()==1 && period[0]=="NO"){
     getValue("")->setPeriodicity(false);
-  } else if(period.size()==2 && Tools::convert(period[0],min) && Tools::convert(period[1],min)){
+  } else if(period.size()==2 && Tools::convert(period[0],min) && Tools::convert(period[1],max)){
     getValue("")->setPeriodicity(true);
     getValue("")->setDomain(min,max);
-  }
+  } else assert(0);
 
   checkRead();
 
diff --git a/src/FunctionMatheval.cpp b/src/FunctionMatheval.cpp
index 4812ef539e0c1f38b132d117c472a679b96f7284..cb2f9d767acd7f712b16a64172f671d4b4f615e1 100644
--- a/src/FunctionMatheval.cpp
+++ b/src/FunctionMatheval.cpp
@@ -92,10 +92,10 @@ names(getNumberOfArguments())
   if(period.size()==0){
   }else if(period.size()==1 && period[0]=="NO"){
     getValue("")->setPeriodicity(false);
-  } else if(period.size()==2 && Tools::convert(period[0],min) && Tools::convert(period[1],min)){
+  } else if(period.size()==2 && Tools::convert(period[0],min) && Tools::convert(period[1],max)){
     getValue("")->setPeriodicity(true);
     getValue("")->setDomain(min,max);
-  }
+  } else assert(0);
   checkRead();
 
   evaluator=evaluator_create(const_cast<char*>(func.c_str()));