diff --git a/connectivity/nanostack/coap-service/test/coap-service/unittest/stub/mbedtls_stub.c b/connectivity/nanostack/coap-service/test/coap-service/unittest/stub/mbedtls_stub.c
index 403ae9a57a5..7ebe517382c 100644
--- a/connectivity/nanostack/coap-service/test/coap-service/unittest/stub/mbedtls_stub.c
+++ b/connectivity/nanostack/coap-service/test/coap-service/unittest/stub/mbedtls_stub.c
@@ -15,6 +15,7 @@
  * limitations under the License.
  */
 
+#include <string.h> // memset
 #include "mbedtls_stub.h"
 
 mbedtls_stub_def mbedtls_stub;