diff --git a/include/libsigrok/proto.h b/include/libsigrok/proto.h
index 19c1680f..0892b99e 100644
--- a/include/libsigrok/proto.h
+++ b/include/libsigrok/proto.h
@@ -33,7 +33,7 @@ SR_API int sr_analog_to_float(const struct sr_datafeed_analog2 *analog,
 SR_API int sr_analog_float_to_string(float value, int digits, char **result);
 SR_API int sr_analog_unit_to_string(const struct sr_datafeed_analog2 *analog,
 		char **result);
-SR_API void sr_rational_set(struct sr_rational *r, uint64_t p, uint64_t q);
+SR_API void sr_rational_set(struct sr_rational *r, int64_t p, uint64_t q);
 
 /*--- backend.c -------------------------------------------------------------*/
 
diff --git a/src/analog.c b/src/analog.c
index d2677a57..db5ac1cf 100644
--- a/src/analog.c
+++ b/src/analog.c
@@ -273,7 +273,7 @@ SR_API int sr_analog_unit_to_string(const struct sr_datafeed_analog2 *analog,
  * @param p Numerator
  * @param q Denominator
  */
-SR_API void sr_rational_set(struct sr_rational *r, uint64_t p, uint64_t q)
+SR_API void sr_rational_set(struct sr_rational *r, int64_t p, uint64_t q)
 {
 	r->p = p;
 	r->q = q;