diff --git a/al_function.hpp b/al_function.hpp
index a49c451..dcb77be 100644
--- a/al_function.hpp
+++ b/al_function.hpp
@@ -3,14 +3,20 @@
 
 #include <cassert>
 
+#include <limits>
 #include <string>
 
 template<class STORAGE, size_t VALUES_COUNT>
 class Function {
     public:
-        Function(): _function_values(0) {}
-        explicit Function(STORAGE val): _function_values(val) {}
+        Function(): _function_values(0) {
+            static_assert(not std::numeric_limits<STORAGE>::is_signed);
+        }
+        explicit Function(STORAGE val): _function_values(val) {
+            static_assert(not std::numeric_limits<STORAGE>::is_signed);
+        }
         explicit Function(std::string val): _function_values(0) {
+            static_assert(not std::numeric_limits<STORAGE>::is_signed);
             assert(("bad input string size", val.size() == VALUES_COUNT));
             STORAGE cur_dig = 1;
             for (auto it = val.rbegin(); it != val.rend(); ++it) {