diff --git a/libc/bionic/ftruncate.cpp b/libc/bionic/ftruncate.cpp
index c073f2d43c86695fc33558b3d8b0c57b0d9a3658..9936df0426418b96a9e47c3b87a4e0301e28167b 100644
--- a/libc/bionic/ftruncate.cpp
+++ b/libc/bionic/ftruncate.cpp
@@ -18,11 +18,14 @@
 #include <sys/cdefs.h>
 #include <unistd.h>
 
-#if !defined(__USE_FILE_OFFSET64) && !defined(__LP64__)
+#if !defined(__LP64__)
+static_assert(sizeof(off_t) == 4,
+              "libc can't be built with _FILE_OFFSET_BITS=64.");
+
 // The kernel's implementation of ftruncate uses an unsigned long for the length
 // parameter, so it will not catch negative values. On the other hand
 // ftruncate64 does check for this, so just forward the call.
 int ftruncate(int filedes, off_t length) {
   return ftruncate64(filedes, length);
 }
-#endif
+#endif  // !defined(__LP64__)
diff --git a/tests/unistd_test.cpp b/tests/unistd_test.cpp
index a21578a899750b5f1938093a7d54127cb44da6ff..79c16d7eb882552667169f6ed2b86249d5b0c3c2 100644
--- a/tests/unistd_test.cpp
+++ b/tests/unistd_test.cpp
@@ -179,10 +179,8 @@ TEST(unistd, ftruncate64) {
 TEST(unistd, ftruncate_negative) {
   TemporaryFile tf;
   errno = 0;
-  int rc = ftruncate(tf.fd, -123);
-  int err = errno;
-  ASSERT_EQ(-1, rc);
-  ASSERT_EQ(EINVAL, err);
+  ASSERT_EQ(-1, ftruncate(tf.fd, -123));
+  ASSERT_EQ(EINVAL, errno);
 }
 
 static bool g_pause_test_flag = false;