diff --git a/tools/misc/xen-mfndump.c b/tools/misc/xen-mfndump.c
index 88cfafa44e..dea9fa9e0c 100644
--- a/tools/misc/xen-mfndump.c
+++ b/tools/misc/xen-mfndump.c
@@ -30,7 +30,8 @@ int help_func(int argc, char *argv[])
 
 int dump_m2p_func(int argc, char *argv[])
 {
-    unsigned long i, max_mfn;
+    unsigned long i;
+    long max_mfn;
     xen_pfn_t *m2p_table;
 
     if ( argc > 0 )
@@ -41,6 +42,12 @@ int dump_m2p_func(int argc, char *argv[])
 
     /* Map M2P and obtain gpfn */
     max_mfn = xc_maximum_ram_page(xch);
+    if ( max_mfn < 0 )
+    {
+        ERROR("Failed to get the maximum mfn");
+        return -1;
+    }
+
     if ( !(m2p_table = xc_map_m2p(xch, max_mfn, PROT_READ, NULL)) )
     {
         ERROR("Failed to map live M2P table");