diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp
--- a/llvm/lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp
@@ -96,25 +96,25 @@
   unsigned TargetFlags = MO.getTargetFlags();
 
   switch (TargetFlags) {
-    case WebAssemblyII::MO_NO_FLAG:
-      break;
-    case WebAssemblyII::MO_GOT_TLS:
-      Kind = MCSymbolRefExpr::VK_WASM_GOT_TLS;
-      break;
-    case WebAssemblyII::MO_GOT:
-      Kind = MCSymbolRefExpr::VK_GOT;
-      break;
-    case WebAssemblyII::MO_MEMORY_BASE_REL:
-      Kind = MCSymbolRefExpr::VK_WASM_MBREL;
-      break;
-    case WebAssemblyII::MO_TLS_BASE_REL:
-      Kind = MCSymbolRefExpr::VK_WASM_TLSREL;
-      break;
-    case WebAssemblyII::MO_TABLE_BASE_REL:
-      Kind = MCSymbolRefExpr::VK_WASM_TBREL;
-      break;
-    default:
-      llvm_unreachable("Unknown target flag on GV operand");
+  case WebAssemblyII::MO_NO_FLAG:
+    break;
+  case WebAssemblyII::MO_GOT_TLS:
+    Kind = MCSymbolRefExpr::VK_WASM_GOT_TLS;
+    break;
+  case WebAssemblyII::MO_GOT:
+    Kind = MCSymbolRefExpr::VK_GOT;
+    break;
+  case WebAssemblyII::MO_MEMORY_BASE_REL:
+    Kind = MCSymbolRefExpr::VK_WASM_MBREL;
+    break;
+  case WebAssemblyII::MO_TLS_BASE_REL:
+    Kind = MCSymbolRefExpr::VK_WASM_TLSREL;
+    break;
+  case WebAssemblyII::MO_TABLE_BASE_REL:
+    Kind = MCSymbolRefExpr::VK_WASM_TBREL;
+    break;
+  default:
+    llvm_unreachable("Unknown target flag on GV operand");
   }
 
   const MCExpr *Expr = MCSymbolRefExpr::create(Sym, Kind, Ctx);
@@ -291,8 +291,7 @@
   auto RegOpcode = OutMI.getOpcode();
   auto StackOpcode = WebAssembly::getStackOpcode(RegOpcode);
   assert(StackOpcode != -1 && "Failed to stackify instruction");
-  OutMI.setOpcode(StackOpcode);
-
+  
   // Remove register operands.
   for (auto I = OutMI.getNumOperands(); I; --I) {
     auto &MO = OutMI.getOperand(I - 1);
@@ -300,4 +299,11 @@
       OutMI.erase(&MO);
     }
   }
+
+  // CALL_S should at least have 1 operand, otherwise this code is invalid.
+  if (StackOpcode == WebAssembly::CALL_S && OutMI.getNumOperands() == 0) {
+    StackOpcode = WebAssembly::UNREACHABLE;
+  }
+
+  OutMI.setOpcode(StackOpcode);
 }
diff --git a/llvm/test/CodeGen/WebAssembly/call.ll b/llvm/test/CodeGen/WebAssembly/call.ll
--- a/llvm/test/CodeGen/WebAssembly/call.ll
+++ b/llvm/test/CodeGen/WebAssembly/call.ll
@@ -2,6 +2,7 @@
 ; RUN: llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -wasm-keep-registers -fast-isel -fast-isel-abort=1 -mattr=+sign-ext,+simd128 | FileCheck --check-prefixes=CHECK,NO-TAIL %s
 ; RUN: llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -wasm-keep-registers -mattr=+sign-ext,+simd128,+tail-call | FileCheck --check-prefixes=CHECK,SLOW-TAIL %s
 ; RUN: llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -wasm-keep-registers -fast-isel -fast-isel-abort=1 -mattr=+sign-ext,+simd128,+tail-call | FileCheck --check-prefixes=CHECK,FAST-TAIL %s
+; RUN: llc < %s -asm-verbose=false -O1 | FileCheck --check-prefixes=OPT %s
 
 ; Test that basic call operations assemble as expected.
 
@@ -236,6 +237,34 @@
   ret void
 }
 
+; OPT-LABEL: call_invalid:
+; OPT-NEXT: .functype	call_invalid () -> ()
+; OPT-NEXT: .local  	i32
+; OPT-NEXT: global.get	__stack_pointer
+; OPT-NEXT: i32.const	16
+; OPT-NEXT: i32.sub 
+; OPT-NEXT: local.tee	0
+; OPT-NEXT: global.set	__stack_pointer
+; OPT-NEXT: local.get	0
+; OPT-NEXT: i32.const	10
+; OPT-NEXT: i32.store	12
+; OPT-NEXT: local.get	0
+; OPT-NEXT: i32.const	12
+; OPT-NEXT: i32.add 
+; OPT-NEXT: unreachable
+; OPT-NEXT: local.get	0
+; OPT-NEXT: i32.const	16
+; OPT-NEXT: i32.add 
+; OPT-NEXT: global.set	__stack_pointer
+; OPT-NEXT: end_function
+define void @call_invalid() {
+entry:
+  %a = alloca i32, align 4
+  store i32 10, ptr %a, align 4
+  call void %a()
+  ret void
+}
+
 ; TODO: test the following:
 ;  - More argument combinations.
 ;  - Tail call.