diff --git a/src/core/thread/router_table.cpp b/src/core/thread/router_table.cpp
index c5f5cfd33..1f9c5af00 100644
--- a/src/core/thread/router_table.cpp
+++ b/src/core/thread/router_table.cpp
@@ -241,7 +241,7 @@ Router *RouterTable::Allocate(uint8_t aRouterId)
 {
     Router *rval = NULL;
 
-    VerifyOrExit(!IsAllocated(aRouterId) && mRouterIdReuseDelay[aRouterId] == 0);
+    VerifyOrExit(aRouterId <= Mle::kMaxRouterId && !IsAllocated(aRouterId) && mRouterIdReuseDelay[aRouterId] == 0);
 
     mAllocatedRouterIds[aRouterId / 8] |= 1 << (aRouterId % 8);
     UpdateAllocation();