diff --git a/library/constant_time.c b/library/constant_time.c
index 9779712..5bbdf1c 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -77,6 +77,8 @@
 #endif
 }
 
+#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
+
 size_t mbedtls_cf_size_mask( size_t value )
 {
     /* MSVC has a warning about unary minus on unsigned integer types,
@@ -91,6 +93,8 @@
 #endif
 }
 
+#endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
+
 #if defined(MBEDTLS_BIGNUM_C)
 
 mbedtls_mpi_uint mbedtls_cf_mpi_uint_mask( mbedtls_mpi_uint value )