diff --git a/include/proto/sample.h b/include/proto/sample.h
index db10537..20c2954 100644
--- a/include/proto/sample.h
+++ b/include/proto/sample.h
@@ -39,5 +39,6 @@
 const char *sample_ckp_names(unsigned int use);
 struct sample_fetch *find_sample_fetch(const char *kw, int len);
 int smp_resolve_args(struct proxy *p);
+int smp_expr_output_type(struct sample_expr *expr);
 
 #endif /* _PROTO_SAMPLE_H */