diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index 48bd49a3..ae0a89cd 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -1319,28 +1319,29 @@ ac_c_conftest_c99_main='
   if (test_restrict ("String literal") == 0)
     success = true;
   char *restrict newvar = "Another string";
 
   // Check varargs.
   success &= test_varargs ("s, d'\'' f .", "string", 65, 34.234);
   test_varargs_macros ();
 
   // Check flexible array members.
   struct incomplete_array *ia =
     malloc (sizeof (struct incomplete_array) + (sizeof (double) * 10));
   ia->datasize = 10;
   for (int i = 0; i < ia->datasize; ++i)
     ia->data[i] = i * 1.234;
+  free(ia);
 
   // Check named initializers.
   struct named_init ni = {
     .number = 34,
     .name = L"Test wide string",
     .average = 543.34343,
   };
 
   ni.number = 58;
 
   int dynamic_array[ni.number];
   dynamic_array[0] = argv[0][0];
   dynamic_array[ni.number - 1] = 543;