diff --git a/smnp/runtime/evaluators/expression.py b/smnp/runtime/evaluators/expression.py
index e0758fa..2418f70 100644
--- a/smnp/runtime/evaluators/expression.py
+++ b/smnp/runtime/evaluators/expression.py
@@ -50,3 +50,15 @@ def expressionEvaluator(doAssert=False):
 
 
     return evaluateExpression
+
+
+def expressionEvaluatorWithMatcher(matcher, exceptionProvider, doAssert=True):
+    def evaluate(node, environment):
+        value = expressionEvaluator(doAssert=doAssert)(node, environment).value
+
+        if not matcher.match(value):
+            raise exceptionProvider(value)
+
+        return value
+
+    return evaluate
\ No newline at end of file
diff --git a/smnp/runtime/evaluators/extend.py b/smnp/runtime/evaluators/extend.py
index 81fb45d..7f8a546 100644
--- a/smnp/runtime/evaluators/extend.py
+++ b/smnp/runtime/evaluators/extend.py
@@ -1,8 +1,7 @@
 from smnp.ast.node.none import NoneNode
 from smnp.function.signature import signature
 from smnp.runtime.evaluator import Evaluator
-from smnp.runtime.tools.signature import argumentsNodeToMethodSignature, listSpecifier, mapSpecifier, \
-    evaluateDefaultArguments
+from smnp.runtime.tools.signature import argumentsNodeToMethodSignature, listSpecifier, mapSpecifier
 from smnp.type.model import Type
 from smnp.type.signature.matcher.type import ofType
 
@@ -33,9 +32,8 @@ class ExtendEvaluator(Evaluator):
     @classmethod
     def _evaluateMethodDefinition(cls, node, environment, type, variable):
         name = node.name.value
-        signature = argumentsNodeToMethodSignature(node.arguments)
+        defaultArguments, signature = argumentsNodeToMethodSignature(node.arguments, environment)
         arguments = [arg.variable.value for arg in node.arguments]
-        defaultArguments = evaluateDefaultArguments(node.arguments, environment)
         body = node.body
         environment.addCustomMethod(type, variable, name, signature, arguments, body, defaultArguments)
 
diff --git a/smnp/runtime/evaluators/function.py b/smnp/runtime/evaluators/function.py
index 39fc792..6b59ca1 100644
--- a/smnp/runtime/evaluators/function.py
+++ b/smnp/runtime/evaluators/function.py
@@ -3,7 +3,7 @@ from smnp.runtime.evaluator import Evaluator, evaluate
 from smnp.runtime.evaluators.expression import expressionEvaluator
 from smnp.runtime.evaluators.iterable import abstractIterableEvaluator
 from smnp.runtime.tools.error import updatePos
-from smnp.runtime.tools.signature import argumentsNodeToMethodSignature, evaluateDefaultArguments
+from smnp.runtime.tools.signature import argumentsNodeToMethodSignature
 from smnp.type.model import Type
 
 
@@ -25,9 +25,8 @@ class FunctionDefinitionEvaluator(Evaluator):
     def evaluator(cls, node, environment):
         try:
             name = node.name.value
-            signature = argumentsNodeToMethodSignature(node.arguments)
+            defaultArguments, signature = argumentsNodeToMethodSignature(node.arguments, environment)
             arguments = [ arg.variable.value for arg in node.arguments ]
-            defaultArguments = evaluateDefaultArguments(node.arguments, environment)
             body = node.body
             environment.addCustomFunction(name, signature, arguments, body, defaultArguments)
         except RuntimeException as e:
diff --git a/smnp/runtime/tools/signature.py b/smnp/runtime/tools/signature.py
index f6d8248..3cef1b5 100644
--- a/smnp/runtime/tools/signature.py
+++ b/smnp/runtime/tools/signature.py
@@ -3,7 +3,7 @@ from smnp.ast.node.none import NoneNode
 from smnp.ast.node.type import TypesList
 from smnp.error.runtime import RuntimeException
 from smnp.function.signature import varargSignature, signature, optional
-from smnp.runtime.evaluators.expression import expressionEvaluator
+from smnp.runtime.evaluators.expression import expressionEvaluator, expressionEvaluatorWithMatcher
 from smnp.runtime.tools.error import updatePos
 from smnp.type.model import Type
 from smnp.type.signature.matcher.list import listOfMatchers
@@ -13,15 +13,17 @@ from smnp.type.signature.matcher.type import allTypes, oneOf, ofType
 
 def evaluateDefaultArguments(node, environment):
     defaultValues = { arg.variable.value: expressionEvaluator(doAssert=True)(arg.optionalValue, environment).value for arg in node.children if type(arg.optionalValue) != NoneNode }
+
     return defaultValues
 
 
-def argumentsNodeToMethodSignature(node):
+def argumentsNodeToMethodSignature(node, environment):
     try:
         sign = []
         vararg = None
         argumentsCount = len(node.children)
         checkPositionOfOptionalArguments(node)
+        defaultArgs = {}
         for i, child in enumerate(node.children):
             matchers = {
                 ast.Type: (lambda c: c.type, typeMatcher),
@@ -29,17 +31,22 @@ def argumentsNodeToMethodSignature(node):
                 TypesList: (lambda c: c, multipleTypeMatcher)
             }
             evaluatedMatcher = matchers[type(child.type)][1](matchers[type(child.type)][0](child))
+
             if child.vararg:
                 if i != argumentsCount - 1:
                     raise RuntimeException("Vararg must be the last argument in signature", child.pos)
                 vararg = evaluatedMatcher
             else:
                 if type(child.optionalValue) != NoneNode:
+                    defaultArgs[child.variable.value] = expressionEvaluatorWithMatcher(
+                        evaluatedMatcher,
+                        exceptionProvider=lambda value: RuntimeException(
+                            f"Value '{value.stringify()}' doesn't match declared type: {evaluatedMatcher.string}", child.optionalValue.pos)
+                    )(child.optionalValue, environment)
                     evaluatedMatcher = optional(evaluatedMatcher)
                 sign.append(evaluatedMatcher)
 
-
-        return varargSignature(vararg, *sign, wrapVarargInValue=True) if vararg is not None else signature(*sign)
+        return defaultArgs, (varargSignature(vararg, *sign, wrapVarargInValue=True) if vararg is not None else signature(*sign))
     except RuntimeException as e:
         raise updatePos(e, node)