diff --git a/include/nlohmann/detail/input/numerizer.hpp b/include/nlohmann/detail/input/numerizer.hpp
index b40ef5b5d..de92c0823 100644
--- a/include/nlohmann/detail/input/numerizer.hpp
+++ b/include/nlohmann/detail/input/numerizer.hpp
@@ -27,13 +27,13 @@ struct numerizer
     }
     
     JSON_HEDLEY_NON_NULL(2)
-    static auto strtoull(const char* str, char** endptr, int base)
+    static unsigned long long int strtoull(const char* str, char** endptr, int base)
     {
         return std::strtoull(str, endptr, base);
     }
     
     JSON_HEDLEY_NON_NULL(2)
-    static auto strtoll(const char* str, char** endptr, int base)
+    static long long int strtoll(const char* str, char** endptr, int base)
     {
         return std::strtoll(str, endptr, base);
     }