diff --git a/src/core/mac/sub_mac.cpp b/src/core/mac/sub_mac.cpp
index 647693da1..04178bac6 100644
--- a/src/core/mac/sub_mac.cpp
+++ b/src/core/mac/sub_mac.cpp
@@ -135,11 +135,11 @@ otError SubMac::Enable(void)
     VerifyOrExit(mState == kStateDisabled);
 
     SuccessOrExit(error = otPlatRadioEnable(&GetInstance()));
-    error = otPlatRadioSleep(&GetInstance());
-    assert(error == OT_ERROR_NONE);
+    SuccessOrExit(error = otPlatRadioSleep(&GetInstance()));
     SetState(kStateSleep);
 
 exit:
+    assert(error == OT_ERROR_NONE);
     return error;
 }