diff --git a/src/alloc.c b/src/alloc.c
index 173beba..1d3281e 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -35,12 +35,22 @@ verifying_alloc(void *opaque, void *oldptr, size_t oldsize, size_t newsize)
     }
 
     if (newsize == 0) {
-        void *newptr = ALLOCATOR_CALL(*wrapped, oldptr_actual, oldsize + sizeof(size_t), 0);
+        void *newptr = ALLOCATOR_CALL(
+            *wrapped,
+            oldptr_actual,
+            oldsize == 0 ? 0 : oldsize + sizeof(size_t),
+            0
+        );
         assert(newptr == NULL);
         return newptr;
     }
 
-    size_t *newptr = ALLOCATOR_CALL(*wrapped, oldptr_actual, oldsize + sizeof(size_t), newsize + sizeof(size_t));
+    size_t *newptr = ALLOCATOR_CALL(
+        *wrapped,
+        oldptr_actual,
+        oldsize == 0 ? 0 : oldsize + sizeof(size_t),
+        newsize + sizeof(size_t)
+    );
     if (newptr == NULL) {
         return NULL;
     }