Index: sys/sys/bitstring.h
===================================================================
--- sys/sys/bitstring.h
+++ sys/sys/bitstring.h
@@ -202,6 +202,11 @@
 	bitstr_t _test;
 	int _value, _offset;
 
+	if (_start >= _nbits) {
+		_result = -1;
+		return;
+	}
+
 	if (_nbits > 0) {
 		_curbitstr = _bitstr + _bit_idx(_start);
 		_stopbitstr = _bitstr + _bit_idx(_nbits - 1);
@@ -231,6 +236,11 @@
 	bitstr_t _test;
 	int _value, _offset;
 
+	if (_start >= _nbits) {
+		_result = -1;
+		return;
+	}
+
 	if (_nbits > 0) {
 		_curbitstr = _bitstr + _bit_idx(_start);
 		_stopbitstr = _bitstr + _bit_idx(_nbits - 1);