diff --git a/dlls/iphlpapi/ifenum.c b/dlls/iphlpapi/ifenum.c
index 0a74ef67d3d..36c42fe0b6f 100644
--- a/dlls/iphlpapi/ifenum.c
+++ b/dlls/iphlpapi/ifenum.c
@@ -234,12 +234,14 @@ InterfaceIndexTable *getNonLoopbackInterfaceIndexTable(void)
 
     if (indexes) {
       struct if_nameindex *p;
+      DWORD size = sizeof(InterfaceIndexTable);
 
       for (p = indexes, numInterfaces = 0; p && p->if_name; p++)
         if (!isLoopbackInterface(fd, p->if_name))
           numInterfaces++;
-      ret = HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY,
-       sizeof(InterfaceIndexTable) + (numInterfaces - 1) * sizeof(DWORD));
+      if (numInterfaces > 1)
+        size += (numInterfaces - 1) * sizeof(DWORD);
+      ret = HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, size);
       if (ret) {
         for (p = indexes; p && p->if_name; p++)
           if (!isLoopbackInterface(fd, p->if_name))