diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 01a118ec16..b7c7584f88 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -3835,10 +3835,15 @@ fn (mut c Checker) check_index_type(typ_sym &table.TypeSymbol, index_type table.
 	// println('index expr left=$typ_sym.source_name $node.pos.line_nr')
 	// if typ_sym.kind == .array && (!(table.type_idx(index_type) in table.number_type_idxs) &&
 	// index_type_sym.kind != .enum_) {
-	if typ_sym.kind in [.array, .array_fixed] && !(index_type.is_number() || index_type_sym.kind ==
-		.enum_) {
-		c.error('non-integer index `$index_type_sym.source_name` (array type `$typ_sym.source_name`)',
-			pos)
+	if typ_sym.kind in [.array, .array_fixed, .string, .ustring] {
+		if !(index_type.is_number() || index_type_sym.kind == .enum_) {
+			type_str := if typ_sym.kind in [.string, .ustring] { 'non-integer string index `$index_type_sym.source_name`' } else { 'non-integer index `$index_type_sym.source_name` (array type `$typ_sym.source_name`)' }
+			c.error('$type_str', pos)
+		}
+		if index_type.has_flag(.optional) {
+			type_str := if typ_sym.kind in [.string, .ustring] { '(type `$typ_sym.source_name`)' } else { '(array type `$typ_sym.source_name`)' }
+			c.error('cannot use optional as index $type_str', pos)
+		}
 	}
 }
 
diff --git a/vlib/v/checker/tests/optional_index_err.out b/vlib/v/checker/tests/optional_index_err.out
new file mode 100644
index 0000000000..ba5f4abdd8
--- /dev/null
+++ b/vlib/v/checker/tests/optional_index_err.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/optional_index_err.vv:3:14: error: cannot use optional as index (type `string`)
+    1 | fn main() {
+    2 |     v := 'hello'
+    3 |     println(v[v.last_index('l')])
+      |              ~~~~~~~~~~~~~~~~~~~
+    4 | }
diff --git a/vlib/v/checker/tests/optional_index_err.vv b/vlib/v/checker/tests/optional_index_err.vv
new file mode 100644
index 0000000000..e34bc6f66a
--- /dev/null
+++ b/vlib/v/checker/tests/optional_index_err.vv
@@ -0,0 +1,4 @@
+fn main() {
+    v := 'hello'
+    println(v[v.last_index('l')])
+}
diff --git a/vlib/v/checker/tests/string_index_non_int_err.out b/vlib/v/checker/tests/string_index_non_int_err.out
new file mode 100644
index 0000000000..449e73f13a
--- /dev/null
+++ b/vlib/v/checker/tests/string_index_non_int_err.out
@@ -0,0 +1,20 @@
+vlib/v/checker/tests/string_index_non_int_err.vv:3:14: error: non-integer string index `string`
+    1 | fn main() {
+    2 |     v := 'foo'
+    3 |     println(v['f'])
+      |              ~~~~~
+    4 |     println(v[true])
+    5 |     println(v[[23]])
+vlib/v/checker/tests/string_index_non_int_err.vv:4:14: error: non-integer string index `bool`
+    2 |     v := 'foo'
+    3 |     println(v['f'])
+    4 |     println(v[true])
+      |              ~~~~~~
+    5 |     println(v[[23]])
+    6 | }
+vlib/v/checker/tests/string_index_non_int_err.vv:5:14: error: non-integer string index `[]int`
+    3 |     println(v['f'])
+    4 |     println(v[true])
+    5 |     println(v[[23]])
+      |              ~~~~~~
+    6 | }
diff --git a/vlib/v/checker/tests/string_index_non_int_err.vv b/vlib/v/checker/tests/string_index_non_int_err.vv
new file mode 100644
index 0000000000..6619e471ca
--- /dev/null
+++ b/vlib/v/checker/tests/string_index_non_int_err.vv
@@ -0,0 +1,6 @@
+fn main() {
+    v := 'foo'
+    println(v['f'])
+    println(v[true])
+    println(v[[23]])
+}