diff --git a/libutf/utftorunestr.c b/libutf/utftorunestr.c
index 005fe8a..ae3ad50 100644
--- a/libutf/utftorunestr.c
+++ b/libutf/utftorunestr.c
@@ -1,12 +1,12 @@
 /* See LICENSE file for copyright and license details. */
 #include "../utf.h"
 
-int
+size_t
 utftorunestr(const char *str, Rune *r)
 {
-	int i, n;
+	size_t i, n;
 
-	for(i = 0; (n = chartorune(&r[i], str)) && r[i]; i++)
+	for (i = 0; (n = chartorune(&r[i], str)) && r[i]; i++)
 		str += n;
 
 	return i;
diff --git a/utf.h b/utf.h
index 23a9887..18a41da 100644
--- a/utf.h
+++ b/utf.h
@@ -59,7 +59,7 @@ int isxdigitrune(Rune);
 Rune tolowerrune(Rune);
 Rune toupperrune(Rune);
 
-int utftorunestr(const char*, Rune *);
+size_t utftorunestr(const char *, Rune *);
 
 int fgetrune(Rune *, FILE *);
 int efgetrune(Rune *, FILE *, const char *);