diff --git a/h_malloc.c b/h_malloc.c
index 625ca5f..c661b8b 100644
--- a/h_malloc.c
+++ b/h_malloc.c
@@ -1630,7 +1630,7 @@ EXPORT size_t h_malloc_usable_size(H_MALLOC_USABLE_SIZE_CONST void *p) {
     struct region_allocator *ra = ro.region_allocator;
     mutex_lock(&ra->lock);
     struct region_metadata *region = regions_find(p);
-    if (p == NULL) {
+    if (region == NULL) {
         fatal_error("invalid malloc_usable_size");
     }
     size_t size = region->size;
diff --git a/test/simple-memory-corruption/test_smc.py b/test/simple-memory-corruption/test_smc.py
index 4122d4a..30e3269 100644
--- a/test/simple-memory-corruption/test_smc.py
+++ b/test/simple-memory-corruption/test_smc.py
@@ -145,9 +145,11 @@ class TestSimpleMemoryCorruption(unittest.TestCase):
                          "fatal allocator error: invalid free\n")
 
     def test_uninitialized_malloc_usable_size(self):
-        _stdout, _stderr, returncode = self.run_test(
+        _stdout, stderr, returncode = self.run_test(
             "uninitialized_malloc_usable_size")
-        self.assertEqual(returncode, -11)
+        self.assertEqual(returncode, -6)
+        self.assertEqual(stderr.decode("utf-8"),
+                         "fatal allocator error: invalid malloc_usable_size\n")
 
     def test_uninitialized_realloc(self):
         _stdout, stderr, returncode = self.run_test("uninitialized_realloc")