diff --git a/libs/spandsp/src/spandsp/t38_core.h b/libs/spandsp/src/spandsp/t38_core.h
index 9170ecd4d5..f6b0b12ea0 100644
--- a/libs/spandsp/src/spandsp/t38_core.h
+++ b/libs/spandsp/src/spandsp/t38_core.h
@@ -282,7 +282,7 @@ SPAN_DECLARE(int) t38_core_send_data_multi_field(t38_core_state_t *s, int data_t
     \param len The length of the packet contents.
     \param seq_no The packet sequence number.
     \return 0 for OK, else -1. */
-SPAN_DECLARE(int) t38_core_rx_ifp_packet(t38_core_state_t *s, const uint8_t *buf, int len, uint16_t seq_no);
+SPAN_DECLARE_NONSTD(int) t38_core_rx_ifp_packet(t38_core_state_t *s, const uint8_t *buf, int len, uint16_t seq_no);
 
 /*! Set the method to be used for data rate management, as per the T.38 spec.
     \param s The T.38 context.
diff --git a/libs/spandsp/src/t38_core.c b/libs/spandsp/src/t38_core.c
index a5abc417a6..443608b26d 100644
--- a/libs/spandsp/src/t38_core.c
+++ b/libs/spandsp/src/t38_core.c
@@ -327,7 +327,7 @@ static __inline__ int classify_seq_no_offset(int expected, int actual)
 }
 /*- End of function --------------------------------------------------------*/
 
-SPAN_DECLARE(int) t38_core_rx_ifp_packet(t38_core_state_t *s, const uint8_t *buf, int len, uint16_t seq_no)
+SPAN_DECLARE_NONSTD(int) t38_core_rx_ifp_packet(t38_core_state_t *s, const uint8_t *buf, int len, uint16_t seq_no)
 {
     int i;
     int t30_indicator;