diff --git a/features/mbedtls/inc/mbedtls/config.h b/features/mbedtls/inc/mbedtls/config.h
index 654af99679..142c715479 100644
--- a/features/mbedtls/inc/mbedtls/config.h
+++ b/features/mbedtls/inc/mbedtls/config.h
@@ -294,7 +294,12 @@
  * Requires MBEDTLS_ENTROPY_C, MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES
  *
  */
-//#define MBEDTLS_TEST_NULL_ENTROPY
+#ifndef MBEDTLS_ENTROPY_HARDWARE_ALT
+/* For devices without HW entropy souces, we are forced to use
+ * NULL entropy. */
+#define MBEDTLS_TEST_NULL_ENTROPY
+#define MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES
+#endif // MBEDTLS_ENTROPY_HARDWARE_ALT
 
 /**
  * \def MBEDTLS_ENTROPY_HARDWARE_ALT