--- a/unicode.c
+++ b/unicode.c
@@ -11,11 +11,11 @@
 
 #define FACTOR 1.42
 
-wchar_t *substr (const wchar_t * s, int start, int end)
+wchar_t *substr (const wchar_t * s, size_t start, size_t end)
 {
     wchar_t *str;
 
-    int len = end - start;
+    size_t len = end - start;
 
     if (len < 1)
         return NULL;