diff --git a/testing/file_test/run_test.cpp b/testing/file_test/run_test.cpp
index c01d88176394..db987fe5d1a7 100644
--- a/testing/file_test/run_test.cpp
+++ b/testing/file_test/run_test.cpp
@@ -122,7 +122,7 @@ auto RunTestFile(const FileTestBase& test_base, bool dump_output,
       input_stream = tmpfile();
       fwrite(split->content.c_str(), sizeof(char), split->content.size(),
              input_stream);
-      rewind(input_stream);
+      CARBON_CHECK(!fseek(input_stream, 0, SEEK_SET));
     } else if (!fs->addFile(split->filename, /*ModificationTime=*/0,
                             llvm::MemoryBuffer::getMemBuffer(
                                 split->content, split->filename,