diff --git a/src/core/mac/sub_mac.cpp b/src/core/mac/sub_mac.cpp
index 0217a2d50..0b5f89ad6 100644
--- a/src/core/mac/sub_mac.cpp
+++ b/src/core/mac/sub_mac.cpp
@@ -85,9 +85,10 @@ SubMac::SubMac(Instance &aInstance)
 
 otRadioCaps SubMac::GetCaps(void) const
 {
-    otRadioCaps caps = mRadioCaps;
+    otRadioCaps caps;
 
 #if OPENTHREAD_RADIO || OPENTHREAD_CONFIG_LINK_RAW_ENABLE
+    caps = mRadioCaps;
 
 #if OPENTHREAD_CONFIG_MAC_SOFTWARE_ACK_TIMEOUT_ENABLE
     caps |= OT_RADIO_CAPS_ACK_TIMEOUT;