diff --git a/src/runtime/sys_windows_amd64.s b/src/runtime/sys_windows_amd64.s
index 612f0a474d..43a26497ad 100644
--- a/src/runtime/sys_windows_amd64.s
+++ b/src/runtime/sys_windows_amd64.s
@@ -351,7 +351,7 @@ TEXT runtime·callbackasm1(SB),NOSPLIT,$0
 	ADDQ	$64, SP
 	POPFQ
 
-	MOVL	-8(CX)(DX*1), AX  // return value
+	MOVQ	-8(CX)(DX*1), AX  // return value
 	POPQ	-8(CX)(DX*1)      // restore bytes just after the args
 	RET
 
diff --git a/src/runtime/syscall_windows_test.go b/src/runtime/syscall_windows_test.go
index 3ad6512976..5335c12f0f 100644
--- a/src/runtime/syscall_windows_test.go
+++ b/src/runtime/syscall_windows_test.go
@@ -655,12 +655,16 @@ uintptr_t cfunc(callback f, uintptr_t n) {
 		r   uintptr
 		err syscall.Errno
 	}
+	want := result{
+		// Make it large enough to test issue #29331.
+		r:   (^uintptr(0)) >> 24,
+		err: 333,
+	}
 	c := make(chan result)
 	go func() {
-		r, _, err := proc.Call(cb, 100)
+		r, _, err := proc.Call(cb, want.r)
 		c <- result{r, err.(syscall.Errno)}
 	}()
-	want := result{r: 100, err: 333}
 	if got := <-c; got != want {
 		t.Errorf("got %d want %d", got, want)
 	}