diff --git a/xdiff-interface.c b/xdiff-interface.c
index db6938689f..18e37d2479 100644
--- a/xdiff-interface.c
+++ b/xdiff-interface.c
@@ -195,7 +195,7 @@ void read_mmblob(mmfile_t *ptr, struct object_database *odb,
 }
 
 #define FIRST_FEW_BYTES 8000
-int buffer_is_binary(const char *ptr, unsigned long size)
+int buffer_is_binary(const char *ptr, size_t size)
 {
 	if (FIRST_FEW_BYTES < size)
 		size = FIRST_FEW_BYTES;
diff --git a/xdiff-interface.h b/xdiff-interface.h
index ce54e1c0e0..41fa1d7562 100644
--- a/xdiff-interface.h
+++ b/xdiff-interface.h
@@ -49,7 +49,7 @@ int xdi_diff_outf(mmfile_t *mf1, mmfile_t *mf2,
 int read_mmfile(mmfile_t *ptr, const char *filename);
 void read_mmblob(mmfile_t *ptr, struct object_database *odb,
 		 const struct object_id *oid);
-int buffer_is_binary(const char *ptr, unsigned long size);
+int buffer_is_binary(const char *ptr, size_t size);
 
 void xdiff_set_find_func(xdemitconf_t *xecfg, const char *line, int cflags);
 void xdiff_clear_find_func(xdemitconf_t *xecfg);