diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
@@ -54,6 +54,11 @@
 
     ForCodeSize = MF.getFunction().hasOptSize();
     Subtarget = &MF.getSubtarget<WebAssemblySubtarget>();
+
+    // Wasm64 is not fully supported right now (and is not specified)
+    if (Subtarget->hasAddr64())
+      report_fatal_error("64-bit WebAssembly (wasm64) is not supported");
+
     return SelectionDAGISel::runOnMachineFunction(MF);
   }
 
diff --git a/llvm/test/CodeGen/WebAssembly/cpus.ll b/llvm/test/CodeGen/WebAssembly/cpus.ll
--- a/llvm/test/CodeGen/WebAssembly/cpus.ll
+++ b/llvm/test/CodeGen/WebAssembly/cpus.ll
@@ -1,16 +1,17 @@
 ; This tests that llc accepts all valid WebAssembly CPUs.
 
 ; RUN: llc < %s -asm-verbose=false -mtriple=wasm32-unknown-unknown -mcpu=mvp 2>&1 | FileCheck %s
-; RUN: llc < %s -asm-verbose=false -mtriple=wasm64-unknown-unknown-wasm -mcpu=mvp 2>&1 | FileCheck %s
+; RUN: not llc < %s -asm-verbose=false -mtriple=wasm64-unknown-unknown-wasm -mcpu=mvp 2>&1 | FileCheck %s --check-prefix=WASM64
 ; RUN: llc < %s -asm-verbose=false -mtriple=wasm32-unknown-unknown -mcpu=generic 2>&1 | FileCheck %s
-; RUN: llc < %s -asm-verbose=false -mtriple=wasm64-unknown-unknown-wasm -mcpu=generic 2>&1 | FileCheck %s
+; RUN: not llc < %s -asm-verbose=false -mtriple=wasm64-unknown-unknown-wasm -mcpu=generic 2>&1 | FileCheck %s  --check-prefix=WASM64
 ; RUN: llc < %s -asm-verbose=false -mtriple=wasm32-unknown-unknown -mcpu=bleeding-edge 2>&1 | FileCheck %s
-; RUN: llc < %s -asm-verbose=false -mtriple=wasm64-unknown-unknown-wasm -mcpu=bleeding-edge 2>&1 | FileCheck %s
+; RUN: not llc < %s -asm-verbose=false -mtriple=wasm64-unknown-unknown-wasm -mcpu=bleeding-edge 2>&1 | FileCheck %s  --check-prefix=WASM64
 ; RUN: llc < %s -asm-verbose=false -mtriple=wasm32-unknown-unknown -mcpu=invalidcpu 2>&1 | FileCheck %s --check-prefix=INVALID
-; RUN: llc < %s -asm-verbose=false -mtriple=wasm64-unknown-unknown-wasm -mcpu=invalidcpu 2>&1 | FileCheck %s --check-prefix=INVALID
+; RUN: not llc < %s -asm-verbose=false -mtriple=wasm64-unknown-unknown-wasm -mcpu=invalidcpu 2>&1 | FileCheck %s --check-prefix=WASM64
 
 ; CHECK-NOT: is not a recognized processor for this target
 ; INVALID: {{.+}} is not a recognized processor for this target
+; WASM64: 64-bit WebAssembly (wasm64) is not supported
 
 define i32 @f(i32 %i_like_the_web) {
   ret i32 %i_like_the_web