diff --git a/models/answer.py b/models/answer.py
index 91b31c4..688f941 100644
--- a/models/answer.py
+++ b/models/answer.py
@@ -1,6 +1,6 @@
 from enum import Enum
 from uuid import UUID
-from typing import TypeAlias, Annotated
+from typing import Annotated, Union, Literal
 
 from pydantic import field_validator, field_serializer, Field
 
@@ -31,7 +31,7 @@ class BaseValue(BaseModel):
 
 
 class TextValue(BaseValue):
-    question_type: form.QuestionType = form.QuestionType.text
+    question_type: Literal[form.QuestionType.text] = form.QuestionType.text
     value: str
 
     def validate(self, question: form.TextQuestion) -> None:
@@ -53,7 +53,7 @@ class TextValue(BaseValue):
 
 
 class ScaleValue(BaseValue):
-    question_type: form.QuestionType = form.QuestionType.scale
+    question_type: Literal[form.QuestionType.scale] = form.QuestionType.scale
     value: int
 
     def validate(self, question: form.ScaleQuestion) -> None:
@@ -62,13 +62,17 @@ class ScaleValue(BaseValue):
 
 
 class SelectorValue(BaseValue):
-    question_type: form.QuestionType = form.QuestionType.selector
+    question_type: Literal[form.QuestionType.selector] = form.QuestionType.selector
     values: set[int]
 
+    @field_serializer("values")
+    def serialize_values(self, values: set[int]):
+        return list(values)
+
     def validate(self, question: form.SelectorQuestion) -> None:
         min_values = max(question.min_values, 1) if question.min_values else 1
         max_values = (
-            min(question.max_values, question.options)
+            min(question.max_values, len(question.options))
             if question.max_values
             else len(question.options)
         )
@@ -79,7 +83,7 @@ class SelectorValue(BaseValue):
             raise ValueError(AnswerError.TOO_MANY_SELECTED.value)
 
 
-Value: TypeAlias = SelectorValue | TextValue | ScaleValue
+Value = Annotated[Union[SelectorValue, TextValue, ScaleValue], Field(discriminator='question_type')]
 
 
 class AnswerData(BaseModel):
diff --git a/models/form.py b/models/form.py
index 16ff761..0cf199f 100644
--- a/models/form.py
+++ b/models/form.py
@@ -1,5 +1,5 @@
 from enum import IntEnum, Enum
-from typing import TypeAlias
+from typing import Annotated, Union, Literal
 from uuid import UUID, uuid4
 
 from pydantic import Field, field_validator, field_serializer
@@ -16,6 +16,7 @@ class FormError(Enum):
     SIMMILAR_ID_ERR = "All questions must have different id's"
     SCALE_MIN_VALUE_ERR = "min_value must be in range from 0 to 1"
     SCALE_MAX_VALUE_ERR = "max_value must be in range from 2 to 10"
+    EMPTY_OPTIONS_ERR = "Options field cannot be empty"
 
 
 class QuestionType(IntEnum):
@@ -44,11 +45,11 @@ class BaseQuestion(BaseModel):
 
 class Option(BaseModel):
     label: str
-    image_url: str | None
+    image_url: str | None = None
 
 
 class TextQuestion(BaseQuestion):
-    question_type: QuestionType = QuestionType.text
+    question_type: Literal[QuestionType.text] = QuestionType.text
     validator: TextValidator | None = None
     min_length: int | None = None
     max_length: int | None = None
@@ -80,7 +81,7 @@ class TextQuestion(BaseQuestion):
 
 
 class ScaleQuestion(BaseQuestion):
-    question_type: QuestionType = QuestionType.scale
+    question_type: Literal[QuestionType.scale] = QuestionType.scale
     min_value: int
     min_label: str | None = None
     max_value: int
@@ -102,17 +103,22 @@ class ScaleQuestion(BaseQuestion):
 
 
 class SelectorQuestion(BaseQuestion):
-    question_type: QuestionType = QuestionType.selector
+    question_type: Literal[QuestionType.selector] = QuestionType.selector
     min_values: int = 1
     max_values: int | None = None
-    options: list[Option] = []
+    options: list[Option]
+
+    @field_validator("options")
+    @classmethod
+    def validate_options(cls, v, info):
+        if len(v) < 1:
+            raise ValueError(FormError.EMPTY_OPTIONS_ERR.value)
+        return v
 
     @field_validator("min_values")
     @classmethod
     def validate_min_values(cls, v, info):
-        options = info.data.get("options")
-        options = [] if not options else options
-        if v is not None and (v < 1 or v > len(options)):
+        if v is not None and v < 1:
             raise ValueError(FormError.MIN_VALUES_ERR.value)
         return v
 
@@ -120,13 +126,12 @@ class SelectorQuestion(BaseQuestion):
     @classmethod
     def validate_max_values(cls, v, info):
         min_values = info.data.get("min_values")
-        options = info.data.get("options")
-        if v is not None and (v > len(options) or min_values > v):
+        if v is not None and min_values > v:
             raise ValueError(FormError.MAX_VALUES_ERR.value)
         return v
 
 
-Question: TypeAlias = SelectorQuestion | TextQuestion | ScaleQuestion
+Question = Annotated[Union[SelectorQuestion, TextQuestion, ScaleQuestion], Field(discriminator='question_type')]
 
 
 class Page(BaseModel):