diff --git a/modules/gmt/modgmt_func.cpp b/modules/gmt/modgmt_func.cpp
index b37e536..30e5b36 100644
--- a/modules/gmt/modgmt_func.cpp
+++ b/modules/gmt/modgmt_func.cpp
@@ -22,7 +22,7 @@ class Base2Something: public OBTypeM<Func,O...>
  Base2Something(const ObjectBase* arg):OBTypeM<Func,O...>(arg) {};
  Base2Something(const ObjectList* input, const std::string& name):OBTypeM<Func,O...>(input->Get(name)){};
  Base2Something(const ObjectList* input, const ObjectList::ListValues::size_type i):OBTypeM<Func,O...>((i<input->Size())?input->At(i):0){}; // Check index, because At is not safe
- auto operator ()(bool* b) -> decltype(this->OBTypeM<Func,O...>::template operator()<bool*>(b)) const {return OBTypeM<Func,O...>::operator ()(b);}
+ auto operator ()(bool* b) const -> decltype(this->OBTypeM<Func,O...>::template operator()<bool*>(b)) {return OBTypeM<Func,O...>::operator ()(b);}
 };
 
 // Converting Int, Real or String to double
@@ -30,13 +30,13 @@ template<class O>
 class DoubleConv
 {
  public:
- double operator ()(const O* q, bool* suc) {return q->Value();}
+ double operator ()(const O* q, bool* suc) const {return q->Value();}
 };
 template<>
 class DoubleConv<ObjectString>
 {
  public:
- double operator ()(const ObjectString* q, bool* suc) {double d=0; *suc=(*suc) && str2double(q->Value(),&d); return d;}
+ double operator ()(const ObjectString* q, bool* suc) const {double d=0; *suc=(*suc) && str2double(q->Value(),&d); return d;}
 };
 typedef Base2Something<DoubleConv,ObjectReal,ObjectInt,ObjectString> Base2Double;