Index: libs/python/src/converter/builtin_converters.cpp
===================================================================
--- libs/python/src/converter/builtin_converters.cpp	(revision 53980)
+++ libs/python/src/converter/builtin_converters.cpp	(working copy)
@@ -155,10 +155,21 @@
   {
       static T extract(PyObject* intermediate)
       {
-          return numeric_cast<T>(
-              PyLong_Check(intermediate)
-              ? PyLong_AsUnsignedLong(intermediate)
-              : PyInt_AS_LONG(intermediate));
+          if (PyLong_Check(intermediate)) {
+              unsigned long result = PyLong_AsUnsignedLong(intermediate);
+              if (PyErr_Occurred())
+                  throw_error_already_set();
+              return numeric_cast<T>(result);
+          } else {
+              long result = PyInt_AS_LONG(intermediate);
+              if (PyErr_Occurred())
+                  throw_error_already_set();
+              if (result < 0) {
+                  PyErr_SetString(PyExc_ValueError, "argument must not be negative");
+                  throw_error_already_set();
+              }
+              return numeric_cast<T>(result);
+          }
       }
   };