diff --git a/include/mbedtls/entropy_poll.h b/include/mbedtls/entropy_poll.h
index dc11911..3fcfef2 100644
--- a/include/mbedtls/entropy_poll.h
+++ b/include/mbedtls/entropy_poll.h
@@ -43,6 +43,14 @@
 #define MBEDTLS_ENTROPY_MIN_HARDCLOCK     4     /**< Minimum for mbedtls_timing_hardclock()        */
 #define MBEDTLS_ENTROPY_MIN_HARDWARE     32     /**< Minimum for the hardware source */
 
+/**
+ * \brief           Entropy poll callback that provides 0 entropy.
+ */
+#if defined(MBEDTLS_TEST_WO_ENTROPY)
+    int mbedtls_zero_entropy_poll( void *data,
+                                unsigned char *output, size_t len, size_t *olen );
+#endif
+
 #if !defined(MBEDTLS_NO_PLATFORM_ENTROPY)
 /**
  * \brief           Platform-specific entropy poll callback