diff --git a/src/evalvars.c b/src/evalvars.c
index d4e8d29..7e6bf7a 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -4223,6 +4223,11 @@
 
     if (varp->v_type == VAR_BOOL)
     {
+	if (is_string_option(varname))
+	{
+	    emsg(_(e_string_required));
+	    return;
+	}
 	numval = (long)varp->vval.v_number;
 	strval = (char_u *)"0";  // avoid using "false"
     }
diff --git a/src/option.c b/src/option.c
index 1b4b9da..8286713 100644
--- a/src/option.c
+++ b/src/option.c
@@ -4478,6 +4478,20 @@
 }
 #endif
 
+#if defined(FEAT_EVAL) || defined(PROTO)
+/*
+ * Return TRUE if "name" is a string option.
+ * Returns FALSE if option "name" does not exist.
+ */
+    int
+is_string_option(char_u *name)
+{
+    int idx = findoption(name);
+
+    return idx >= 0 && (options[idx].flags & P_STRING);
+}
+#endif
+
 /*
  * Translate a string like "t_xx", "<t_xx>" or "<S-Tab>" to a key number.
  * When "has_lt" is true there is a '<' before "*arg_arg".
diff --git a/src/proto/option.pro b/src/proto/option.pro
index c64672d..ca150b6 100644
--- a/src/proto/option.pro
+++ b/src/proto/option.pro
@@ -43,6 +43,7 @@
 char_u *get_highlight_default(void);
 char_u *get_encoding_default(void);
 int is_option_allocated(char *name);
+int is_string_option(char_u *name);
 int makeset(FILE *fd, int opt_flags, int local_only);
 int makefoldset(FILE *fd);
 void clear_termoptions(void);
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 0243595..b8e2ca7 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -3719,6 +3719,7 @@
   v9.CheckDefAndScriptFailure(['setwinvar("a", "b", 1)'], ['E1013: Argument 1: type mismatch, expected number but got string', 'E1210: Number required for argument 1'])
   v9.CheckDefAndScriptFailure(['setwinvar(1, 2, "c")'], ['E1013: Argument 2: type mismatch, expected string but got number', 'E1174: String required for argument 2'])
   assert_fails('setwinvar(1, "", 10)', 'E461: Illegal variable name')
+  assert_fails('setwinvar(0, "&rulerformat", true)', 'E928:')
 enddef
 
 def Test_sha256()
diff --git a/src/version.c b/src/version.c
index 74fb0e1..2becd39 100644
--- a/src/version.c
+++ b/src/version.c
@@ -736,6 +736,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    90,
+/**/
     89,
 /**/
     88,