diff --git a/src/cmd/gc/sys.go b/src/cmd/gc/sys.go
index 37c523d992a..8e2e6ac76d4 100644
--- a/src/cmd/gc/sys.go
+++ b/src/cmd/gc/sys.go
@@ -76,7 +76,7 @@ export func	goexit();
 export func	readfile(string) (string, bool);	// read file into string; boolean status
 export func	writefile(string, string) (bool);	// write string into file; boolean status
 export func	bytestorune(*byte, int32, int32) (int32, int32);	// convert bytes to runes
-export func	stringtorune(string, int32, int32) (int32, int32);	// convert bytes to runes
+export func	stringtorune(string, int32) (int32, int32);	// convert bytes to runes
 
 export func	exit(int32);
 
diff --git a/src/cmd/gc/sysimport.c b/src/cmd/gc/sysimport.c
index c3b7cbfc1f9..1ef5d20be0d 100644
--- a/src/cmd/gc/sysimport.c
+++ b/src/cmd/gc/sysimport.c
@@ -1,4 +1,4 @@
-char *sysimport =
+char *sysimport = 
 	"package sys\n"
 	"type sys.any any\n"
 	"type sys.uint32 uint32\n"
@@ -66,7 +66,7 @@ char *sysimport =
 	"export func sys.readfile (? sys.string) (? sys.string, ? sys.bool)\n"
 	"export func sys.writefile (? sys.string, ? sys.string) (? sys.bool)\n"
 	"export func sys.bytestorune (? *sys.uint8, ? sys.int32, ? sys.int32) (? sys.int32, ? sys.int32)\n"
-	"export func sys.stringtorune (? sys.string, ? sys.int32, ? sys.int32) (? sys.int32, ? sys.int32)\n"
+	"export func sys.stringtorune (? sys.string, ? sys.int32) (? sys.int32, ? sys.int32)\n"
 	"export func sys.exit (? sys.int32)\n"
 	"\n"
 	"$$\n";
diff --git a/src/runtime/rune.c b/src/runtime/rune.c
index d705a5e36f7..5738ca3646a 100644
--- a/src/runtime/rune.c
+++ b/src/runtime/rune.c
@@ -235,9 +235,9 @@ sys·bytestorune(byte *str, int32 off, int32 length, int32 outrune, int32 outcou
 }
 
 void
-sys·stringtorune(string str, int32 off, int32 length, int32 outrune, int32 outcount)
+sys·stringtorune(string str, int32 off, int32 outrune, int32 outcount)
 {
-	outcount = charntorune(&outrune, str->str + off, length);
+	outcount = charntorune(&outrune, str->str + off, str->len - off);
 	FLUSH(&outrune);
 	FLUSH(&outcount);
 }
diff --git a/src/runtime/runtime.h b/src/runtime/runtime.h
index f590472439b..f182aebae79 100644
--- a/src/runtime/runtime.h
+++ b/src/runtime/runtime.h
@@ -293,4 +293,4 @@ void	sys·intstring(int64, string);
  */
 void	sys·readfile(string, string, bool);
 void	sys·bytestorune(byte*, int32, int32, int32, int32);
-void	sys·stringtorune(string, int32, int32, int32, int32);
+void	sys·stringtorune(string, int32, int32, int32);
diff --git a/test/utf.go b/test/utf.go
index 206f67c7314..f38f4edd8c8 100644
--- a/test/utf.go
+++ b/test/utf.go
@@ -21,7 +21,7 @@ func main() {
 	var l = len(s);
 	for w, i, j := 0,0,0; i < l; i += w {
 		var r int32;
-		r, w = sys.stringtorune(s, i, l);
+		r, w = sys.stringtorune(s, i);
 		if w == 0 { panic("zero width in string") }
 		if r != chars[j] { panic("wrong value from string") }
 		j++;