diff --git a/lib/streambuf.c b/lib/streambuf.c
index 4b6c0bec6..e3f36e17a 100644
--- a/lib/streambuf.c
+++ b/lib/streambuf.c
@@ -177,7 +177,7 @@ char *streambuf_getline(struct streambuf *b) {
 	return s;
 }
 
-unsigned int streambuf_bufsize(struct streambuf *b) {
+size_t streambuf_bufsize(struct streambuf *b) {
 	return b->buf->len;
 }
 
diff --git a/lib/streambuf.h b/lib/streambuf.h
index e47810b5d..ea4919936 100644
--- a/lib/streambuf.h
+++ b/lib/streambuf.h
@@ -41,7 +41,7 @@ void streambuf_destroy(struct streambuf *);
 int streambuf_writeable(struct streambuf *);
 int streambuf_readable(struct streambuf *);
 char *streambuf_getline(struct streambuf *);
-unsigned int streambuf_bufsize(struct streambuf *);
+size_t streambuf_bufsize(struct streambuf *);
 void streambuf_printf(struct streambuf *, const char *, ...) __attribute__ ((format (printf, 2, 3)));
 void streambuf_vprintf(struct streambuf *, const char *, va_list);
 void streambuf_write(struct streambuf *, const char *, unsigned int);