diff --git a/src/quicksort.c b/src/quicksort.c
index 03c0899..8bb2883 100644
--- a/src/quicksort.c
+++ b/src/quicksort.c
@@ -99,21 +99,22 @@ int benchmark(int argc, char **argv) {
       break;
     case 'n':
       n = atoi(optarg);
-      if (n <= 0) {
-        goto usage;
-      }
       break;
     case 't':
       nthreads = atoi(optarg);
-      if (nthreads <= 0) {
-        goto usage;
-      }
       break;
     default:
       goto usage;
     }
   }
 
+  if (n <= 0) {
+    goto usage;
+  }
+  if (nthreads < 0 && !serial) {
+    goto usage;
+  }
+
   a = malloc(n * sizeof(int));
 
   unsigned long long s = 0;