diff --git a/builtin/string_builder.v b/builtin/string_builder.v
index 6fbc9847d1..5782b8ff42 100644
--- a/builtin/string_builder.v
+++ b/builtin/string_builder.v
@@ -34,3 +34,6 @@ fn (b StringBuilder) cut(n int) {
 	b.len -= n
 }
 
+fn (b mut StringBuilder) free() {
+	C.free(b.buf.data)
+}