diff --git a/library/ecp_invasive.h b/library/ecp_invasive.h
index 3ee238e..31646b9 100644
--- a/library/ecp_invasive.h
+++ b/library/ecp_invasive.h
@@ -95,6 +95,13 @@
 
 #endif /* MBEDTLS_ECP_DP_SECP192R1_ENABLED */
 
+#if defined(MBEDTLS_ECP_DP_SECP521R1_ENABLED)
+
+MBEDTLS_STATIC_TESTABLE
+int ecp_mod_p521_raw(mbedtls_mpi_uint *N_p, size_t N_n);
+
+#endif /* MBEDTLS_ECP_DP_SECP521R1_ENABLED */
+
 #endif /* MBEDTLS_TEST_HOOKS && MBEDTLS_ECP_C */
 
 #endif /* MBEDTLS_ECP_INVASIVE_H */