diff --git a/vl.c b/vl.c
index f5afed4..b9146cf 100644
--- a/vl.c
+++ b/vl.c
@@ -953,8 +953,8 @@
             node_mem[nodenr] = 0;
         } else {
             int64_t sval;
-            sval = strtosz(option, NULL);
-            if (sval < 0) {
+            sval = strtosz(option, &endptr);
+            if (sval < 0 || *endptr) {
                 fprintf(stderr, "qemu: invalid numa mem size: %s\n", optarg);
                 exit(1);
             }