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
@@ -300,4 +300,6 @@
       OutMI.erase(&MO);
     }
   }
+  // CALL_S should at least have 1 operand, otherwise this code is invalid.
+  assert(!(StackOpcode == WebAssembly::CALL_S && OutMI.getNumOperands() == 0));
 }