diff --git a/library/constant_time.c b/library/constant_time.c
index e4185ae..edf2e48 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -160,6 +160,7 @@
  *
  * Constant flow with respect to c.
  */
+MBEDTLS_STATIC_TESTABLE
 unsigned char mbedtls_ct_uchar_mask_of_range( unsigned char low,
                                               unsigned char high,
                                               unsigned char c )