Index: lib/MC/WasmObjectWriter.cpp
===================================================================
--- lib/MC/WasmObjectWriter.cpp
+++ lib/MC/WasmObjectWriter.cpp
@@ -398,8 +398,14 @@
 // Now that the section is complete and we know how big it is, patch up the
 // section size field at the start of the section.
 void WasmObjectWriter::endSection(SectionBookkeeping &Section) {
-  uint64_t Size = W.OS.tell() - Section.PayloadOffset;
-  if (uint32_t(Size) != Size)
+  uint64_t Size = W.OS.tell();
+  // /dev/null doesn't support seek/tell and can report offset of 0.
+  // Simply skip this patching in that case.
+  if (!Size)
+    return;
+
+  Size -= Section.PayloadOffset;
+  if (Size && uint32_t(Size) != Size)
     report_fatal_error("section size does not fit in a uint32_t");
 
   LLVM_DEBUG(dbgs() << "endSection size=" << Size << "\n");
Index: test/MC/WebAssembly/null-output.s
===================================================================
--- /dev/null
+++ test/MC/WebAssembly/null-output.s
@@ -0,0 +1,10 @@
+# RUN: llvm-mc -triple=wasm32-unknown-unknown -filetype=obj -o /dev/null < %s
+
+    .text
+    .section .text.main,"",@
+    .type    main,@function
+main:
+    .functype   main (i32, i32) -> (i32)
+    end_function
+.Lfunc_end0:
+    .size main, .Lfunc_end0-main