diff --git a/libc/bionic/pthread_attr.cpp b/libc/bionic/pthread_attr.cpp
index 2763b0c..dfb740d 100644
--- a/libc/bionic/pthread_attr.cpp
+++ b/libc/bionic/pthread_attr.cpp
@@ -111,7 +111,7 @@
   if ((stack_size & (PAGE_SIZE - 1) || stack_size < PTHREAD_STACK_MIN)) {
     return EINVAL;
   }
-  if ((uint32_t)stack_base & (PAGE_SIZE - 1)) {
+  if (reinterpret_cast<uintptr_t>(stack_base) & (PAGE_SIZE - 1)) {
     return EINVAL;
   }
   attr->stack_base = stack_base;