diff --git a/library/constant_time_internal.h b/library/constant_time_internal.h
index f028622..90d1ac6 100644
--- a/library/constant_time_internal.h
+++ b/library/constant_time_internal.h
@@ -171,6 +171,8 @@
 
 unsigned char mbedtls_ct_base64_enc_char( unsigned char val );
 
+signed char mbedtls_ct_base64_dec_value( unsigned char c );
+
 #endif /* MBEDTLS_BASE64_C */
 
 #if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)