diff --git a/src/routing/global-routing/global-route-manager-impl.cc b/src/routing/global-routing/global-route-manager-impl.cc
index a5e29a059..9587dd441 100644
--- a/src/routing/global-routing/global-route-manager-impl.cc
+++ b/src/routing/global-routing/global-route-manager-impl.cc
@@ -1382,7 +1382,7 @@ GlobalRouteManagerImpl::SPFIntraAddStub (GlobalRoutingLinkRecord *l, SPFVertex*
 // If no such interface is found, return -1 (note:  unit test framework
 // for routing assumes -1 to be a legal return value)
 //
-  uint32_t
+int32_t
 GlobalRouteManagerImpl::FindOutgoingInterfaceId (Ipv4Address a, Ipv4Mask amask)
 {
   NS_LOG_FUNCTION (a << amask);
diff --git a/src/routing/global-routing/global-route-manager-impl.h b/src/routing/global-routing/global-route-manager-impl.h
index 2ae8406e0..3ce2349db 100644
--- a/src/routing/global-routing/global-route-manager-impl.h
+++ b/src/routing/global-routing/global-route-manager-impl.h
@@ -799,7 +799,7 @@ private:
   void SPFIntraAddRouter (SPFVertex* v);
   void SPFIntraAddTransit (SPFVertex* v);
   void SPFIntraAddStub (GlobalRoutingLinkRecord *l, SPFVertex* v);
-  uint32_t FindOutgoingInterfaceId (Ipv4Address a, 
+  int32_t FindOutgoingInterfaceId (Ipv4Address a, 
     Ipv4Mask amask = Ipv4Mask("255.255.255.255"));
 
   // Local cache of the Ipv4GlobalRouting objects, indexed by nodeId