diff --git a/src/eval.c b/src/eval.c
index 81913c7..451bb16 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -2420,9 +2420,9 @@
 {
     char_u	*p;
     int		getnext;
-    int		i;
     exptype_T	type = EXPR_UNKNOWN;
     int		len = 2;
+    int		type_is = FALSE;
 
     /*
      * Get the first variable.
@@ -2431,44 +2431,7 @@
 	return FAIL;
 
     p = eval_next_non_blank(*arg, evalarg, &getnext);
-    switch (p[0])
-    {
-	case '=':   if (p[1] == '=')
-			type = EXPR_EQUAL;
-		    else if (p[1] == '~')
-			type = EXPR_MATCH;
-		    break;
-	case '!':   if (p[1] == '=')
-			type = EXPR_NEQUAL;
-		    else if (p[1] == '~')
-			type = EXPR_NOMATCH;
-		    break;
-	case '>':   if (p[1] != '=')
-		    {
-			type = EXPR_GREATER;
-			len = 1;
-		    }
-		    else
-			type = EXPR_GEQUAL;
-		    break;
-	case '<':   if (p[1] != '=')
-		    {
-			type = EXPR_SMALLER;
-			len = 1;
-		    }
-		    else
-			type = EXPR_SEQUAL;
-		    break;
-	case 'i':   if (p[1] == 's')
-		    {
-			if (p[2] == 'n' && p[3] == 'o' && p[4] == 't')
-			    len = 5;
-			i = p[len];
-			if (!isalnum(i) && i != '_')
-			    type = len == 2 ? EXPR_IS : EXPR_ISNOT;
-		    }
-		    break;
-    }
+    type = get_compare_type(p, &len, &type_is);
 
     /*
      * If there is a comparative operator, use it.
@@ -2482,6 +2445,13 @@
 	if (getnext)
 	    *arg = eval_next_line(evalarg);
 
+	if (vim9script && type_is && (p[len] == '?' || p[len] == '#'))
+	{
+	    semsg(_(e_invexpr2), p);
+	    clear_tv(rettv);
+	    return FAIL;
+	}
+
 	// extra question mark appended: ignore case
 	if (p[len] == '?')
 	{
diff --git a/src/proto/vim9compile.pro b/src/proto/vim9compile.pro
index 4108c04..fd2dd1b 100644
--- a/src/proto/vim9compile.pro
+++ b/src/proto/vim9compile.pro
@@ -16,6 +16,7 @@
 char_u *peek_next_line_from_context(cctx_T *cctx);
 char_u *next_line_from_context(cctx_T *cctx, int skip_comment);
 char_u *to_name_const_end(char_u *arg);
+exptype_T get_compare_type(char_u *p, int *len, int *type_is);
 int assignment_len(char_u *p, int *heredoc);
 void vim9_declare_error(char_u *name);
 int check_vim9_unlet(char_u *name);
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index e29ef5c..550e419 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -831,7 +831,7 @@
 enddef
 
 def Test_expr5_vim9script()
-  # only checks line continuation
+  # check line continuation
   let lines =<< trim END
       vim9script
       let var = 11
@@ -848,6 +848,30 @@
       assert_equal('onetwo', var)
   END
   CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      echo 'abc' is# 'abc'
+  END
+  CheckScriptFailure(lines, 'E15:')
+
+  lines =<< trim END
+      vim9script
+      echo 'abc' is? 'abc'
+  END
+  CheckScriptFailure(lines, 'E15:')
+
+  lines =<< trim END
+      vim9script
+      echo 'abc' isnot# 'abc'
+  END
+  CheckScriptFailure(lines, 'E15:')
+
+  lines =<< trim END
+      vim9script
+      echo 'abc' isnot? 'abc'
+  END
+  CheckScriptFailure(lines, 'E15:')
 enddef
 
 def Test_expr5_float()
diff --git a/src/version.c b/src/version.c
index bb1c4ab..033f360 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1323,
+/**/
     1322,
 /**/
     1321,
diff --git a/src/vim9compile.c b/src/vim9compile.c
index f150904..cf08e95 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -3633,7 +3633,7 @@
     }
 }
 
-    static exptype_T
+    exptype_T
 get_compare_type(char_u *p, int *len, int *type_is)
 {
     exptype_T	type = EXPR_UNKNOWN;