diff --git a/libc/bionic/mmap.cpp b/libc/bionic/mmap.cpp
index 864dea2..febc459 100644
--- a/libc/bionic/mmap.cpp
+++ b/libc/bionic/mmap.cpp
@@ -37,13 +37,14 @@
 
 #define MMAP2_SHIFT 12 // 2**12 == 4096
 
-void* mmap(void* addr, size_t size, int prot, int flags, int fd, long offset) {
+void* mmap(void* addr, size_t size, int prot, int flags, int fd, off_t offset) {
   if (offset & ((1UL << MMAP2_SHIFT)-1)) {
     errno = EINVAL;
     return MAP_FAILED;
   }
 
-  void* result = __mmap2(addr, size, prot, flags, fd, offset >> MMAP2_SHIFT);
+  size_t unsigned_offset = static_cast<size_t>(offset); // To avoid sign extension.
+  void* result = __mmap2(addr, size, prot, flags, fd, unsigned_offset >> MMAP2_SHIFT);
 
   if (result != MAP_FAILED && (flags & (MAP_PRIVATE | MAP_ANONYMOUS)) != 0) {
     ErrnoRestorer errno_restorer;