Index: uspace/lib/cpp/include/__bits/locale/num_get.hpp
===================================================================
--- uspace/lib/cpp/include/__bits/locale/num_get.hpp	(revision bc56f30178279e1e59b8add425abe116ecae93b5)
+++ uspace/lib/cpp/include/__bits/locale/num_get.hpp	(revision f3f41498e327dbcb4cefa602eec09e68135fa2cd)
@@ -303,11 +303,12 @@
                     int olderrno{errno};
                     errno = EOK;
+                    char *endptr = NULL;
 
                     if constexpr (is_signed<BaseType>::value)
-                        res = ::strtoll(base.buffer_, nullptr, num_base);
-                    else
-                        res = ::strtoull(base.buffer_, nullptr, num_base);
-
-                    if (errno != EOK)
+                        res = ::strtoll(base.buffer_, &endptr, num_base);
+                    else
+                        res = ::strtoull(base.buffer_, &endptr, num_base);
+
+                    if (errno != EOK || endptr == base.buffer_)
                         err |= ios_base::failbit;