diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -1061,6 +1061,9 @@
     auto *FuncSection = static_cast<MCSectionWasm *>(Func.Section);
 
     int64_t Size = Layout.getSectionAddressSize(FuncSection);
+    // Wasm functions can't have zero size, they at least need to
+    // encode the numer of locals, even if that number is zero.
+    assert(Size);
     encodeULEB128(Size, W->OS);
     FuncSection->setSectionOffset(W->OS.tell() - Section.ContentsOffset);
     Asm.writeSectionData(W->OS, FuncSection, Layout);
diff --git a/llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp b/llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
--- a/llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
+++ b/llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
@@ -863,7 +863,7 @@
       if (SymName.empty())
         return true;
       auto WasmSym = cast<MCSymbolWasm>(Ctx.getOrCreateSymbol(SymName));
-      if (WasmSym->isDefined()) {
+      if (WasmSym->isDefined() && CurrentState != FunctionStart) {
         // This .functype indicates a start of a function.
         if (ensureEmptyNestingStack())
           return true;
@@ -1060,10 +1060,18 @@
   void doBeforeLabelEmit(MCSymbol *Symbol, SMLoc IDLoc) override {
     // Code below only applies to labels in text sections.
     auto CWS = cast<MCSectionWasm>(getStreamer().getCurrentSection().first);
-    if (!CWS || !CWS->getKind().isText())
+    auto WasmSym = cast<MCSymbolWasm>(Symbol);
+    if (!CWS || !CWS->getKind().isText()) {
+      if (CurrentState == Instructions || CurrentState == FunctionStart ||
+          CurrentState == FunctionLocals) {
+        Parser.Error(IDLoc,
+                     "premature end of function (expected end_function)");
+        CurrentState = EndFunction;
+        pop("end_function", Function);
+      }
       return;
+    }
 
-    auto WasmSym = cast<MCSymbolWasm>(Symbol);
     // Unlike other targets, we don't allow data in text sections (labels
     // declared with .type @object).
     if (WasmSym->getType() == wasm::WASM_SYMBOL_TYPE_DATA) {
@@ -1072,6 +1080,19 @@
       return;
     }
 
+    if (WasmSym->isFunction()) {
+      if (CurrentState == FunctionStart && LastFunctionLabel != WasmSym) {
+        Parser.Error(
+            IDLoc, "previous function not terminated (expected end_function)");
+        pop("end_function", Function);
+      }
+      if (ensureEmptyNestingStack())
+        return;
+      CurrentState = FunctionStart;
+      LastFunctionLabel = WasmSym;
+      push(Function);
+    }
+
     // Start a new section for the next function automatically, since our
     // object writer expects each function to have its own section. This way
     // The user can't forget this "convention".
diff --git a/llvm/test/MC/WebAssembly/empty-function-body.s b/llvm/test/MC/WebAssembly/empty-function-body.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/WebAssembly/empty-function-body.s
@@ -0,0 +1,7 @@
+# RUN: not llvm-mc -triple=wasm32-unknown-unknown -filetype=obj %s -o %t.o 2>&1 | FileCheck %s
+
+# An empty function should produce an error.
+foo:
+  .functype foo () -> ()
+
+# CHECK: empty-function-body.s:8:1: error: Unmatched block construct(s) at function end: function
diff --git a/llvm/test/MC/WebAssembly/empty-function-body2.s b/llvm/test/MC/WebAssembly/empty-function-body2.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/WebAssembly/empty-function-body2.s
@@ -0,0 +1,8 @@
+# RUN: not llvm-mc -triple=wasm32-unknown-unknown -filetype=obj %s -o %t.o 2>&1 | FileCheck %s
+
+# This is the same as empty-function-body.s but with the .functype declaration
+# before the body of the function.
+.functype bar () -> ()
+bar:
+
+# CHECK: empty-function-body2.s:9:1: error: Unmatched block construct(s) at function end: function
diff --git a/llvm/test/MC/WebAssembly/empty-function-body3.s b/llvm/test/MC/WebAssembly/empty-function-body3.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/WebAssembly/empty-function-body3.s
@@ -0,0 +1,23 @@
+# RUN: not llvm-mc -triple=wasm32-unknown-unknown -filetype=obj %s -o %t.o 2>&1 | FileCheck %s
+
+# An empty function should produce an error as soon as a non-function symbol
+# is defined.
+foo:
+  .functype foo () -> ()
+
+.section .data.bar,"",@
+bar:
+# CHECK: empty-function-body3.s:9:1: error: premature end of function (expected end_function)
+  .int32   7
+  .size    bar, 4
+
+.section .text.foo2,"",@
+foo2:
+  .functype foo2 () -> ()
+
+# .. or when another function body is encountered.
+.section .text.bar2,"",@
+.functype bar2 () -> ()
+bar2:
+# CHECK: empty-function-body3.s:21:1: error: previous function not terminated (expected end_function)
+  end_function