diff --git a/libc/bionic/mmap.cpp b/libc/bionic/mmap.cpp
index 84a0f76..28a47cc 100644
--- a/libc/bionic/mmap.cpp
+++ b/libc/bionic/mmap.cpp
@@ -38,14 +38,12 @@
 #define MMAP2_SHIFT 12 // 2**12 == 4096
 
 void* mmap64(void* addr, size_t size, int prot, int flags, int fd, off64_t offset) {
-  if (offset & ((1UL << MMAP2_SHIFT)-1)) {
+  if (offset < 0 || (offset & ((1UL << MMAP2_SHIFT)-1)) != 0) {
     errno = EINVAL;
     return MAP_FAILED;
   }
 
-  uint64_t unsigned_offset = static_cast<uint64_t>(offset); // To avoid sign extension.
-  void* result = __mmap2(addr, size, prot, flags, fd, unsigned_offset >> MMAP2_SHIFT);
-
+  void* result = __mmap2(addr, size, prot, flags, fd, offset >> MMAP2_SHIFT);
   if (result != MAP_FAILED && (flags & (MAP_PRIVATE | MAP_ANONYMOUS)) != 0) {
     ErrnoRestorer errno_restorer;
     madvise(result, size, MADV_MERGEABLE);
@@ -55,5 +53,5 @@
 }
 
 void* mmap(void* addr, size_t size, int prot, int flags, int fd, off_t offset) {
-  return mmap64(addr, size, prot, flags, fd, static_cast<off64_t>(offset) & 0xffffffff);
+  return mmap64(addr, size, prot, flags, fd, static_cast<off64_t>(offset));
 }
diff --git a/tests/Android.mk b/tests/Android.mk
index 4985643..ee22c35 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -61,6 +61,7 @@
     strings_test.cpp \
     stubs_test.cpp \
     sys_epoll_test.cpp \
+    sys_mman_test.cpp \
     sys_resource_test.cpp \
     sys_select_test.cpp \
     sys_sendfile_test.cpp \
diff --git a/tests/sys_mman_test.cpp b/tests/sys_mman_test.cpp
new file mode 100644
index 0000000..57067d7
--- /dev/null
+++ b/tests/sys_mman_test.cpp
@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2014 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <gtest/gtest.h>
+
+#include <sys/mman.h>
+#include <unistd.h>
+
+TEST(sys_mman, mmap_negative) {
+  off_t off = -sysconf(_SC_PAGESIZE); // Aligned but negative.
+  ASSERT_EQ(MAP_FAILED, mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, -1, off));
+}
+
+TEST(sys_mman, mmap64_negative) {
+  off64_t off64 = -sysconf(_SC_PAGESIZE); // Aligned but negative.
+  ASSERT_EQ(MAP_FAILED, mmap64(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, -1, off64));
+}