diff --git a/libsolidity/ast/AST.cpp b/libsolidity/ast/AST.cpp
index d8ad009d1..80f5d6424 100644
--- a/libsolidity/ast/AST.cpp
+++ b/libsolidity/ast/AST.cpp
@@ -297,7 +297,7 @@ ContractDefinition::ContractKind FunctionDefinition::inContractKind() const
 	return contractDef->contractKind();
 }
 
-shared_ptr<FunctionType> FunctionDefinition::functionType(bool _internal) const
+FunctionTypePointer FunctionDefinition::functionType(bool _internal) const
 {
 	if (_internal)
 	{
@@ -338,6 +338,7 @@ shared_ptr<FunctionType> FunctionDefinition::functionType(bool _internal) const
 
 TypePointer FunctionDefinition::type() const
 {
+	solAssert(visibility() != Declaration::Visibility::External, "");
 	return make_shared<FunctionType>(*this);
 }
 
@@ -379,7 +380,7 @@ TypePointer EventDefinition::type() const
 	return make_shared<FunctionType>(*this);
 }
 
-std::shared_ptr<FunctionType> EventDefinition::functionType(bool _internal) const
+FunctionTypePointer EventDefinition::functionType(bool _internal) const
 {
 	if (_internal)
 		return make_shared<FunctionType>(*this);
@@ -484,7 +485,7 @@ TypePointer VariableDeclaration::type() const
 	return annotation().type;
 }
 
-shared_ptr<FunctionType> VariableDeclaration::functionType(bool _internal) const
+FunctionTypePointer VariableDeclaration::functionType(bool _internal) const
 {
 	if (_internal)
 		return {};
diff --git a/libsolidity/ast/AST.h b/libsolidity/ast/AST.h
index ae253f0c8..b648e08b2 100644
--- a/libsolidity/ast/AST.h
+++ b/libsolidity/ast/AST.h
@@ -218,7 +218,7 @@ public:
 
 	/// @param _internal false indicates external interface is concerned, true indicates internal interface is concerned.
 	/// @returns null when it is not accessible as a function.
-	virtual std::shared_ptr<FunctionType> functionType(bool /*_internal*/) const { return {}; }
+	virtual FunctionTypePointer functionType(bool /*_internal*/) const { return {}; }
 
 protected:
 	virtual Visibility defaultVisibility() const { return Visibility::Public; }
@@ -634,7 +634,7 @@ public:
 
 	/// @param _internal false indicates external interface is concerned, true indicates internal interface is concerned.
 	/// @returns null when it is not accessible as a function.
-	virtual std::shared_ptr<FunctionType> functionType(bool /*_internal*/) const override;
+	virtual FunctionTypePointer functionType(bool /*_internal*/) const override;
 
 	virtual FunctionDefinitionAnnotation& annotation() const override;
 
@@ -703,7 +703,7 @@ public:
 
 	/// @param _internal false indicates external interface is concerned, true indicates internal interface is concerned.
 	/// @returns null when it is not accessible as a function.
-	virtual std::shared_ptr<FunctionType> functionType(bool /*_internal*/) const override;
+	virtual FunctionTypePointer functionType(bool /*_internal*/) const override;
 
 	virtual VariableDeclarationAnnotation& annotation() const override;
 
@@ -805,7 +805,7 @@ public:
 	bool isAnonymous() const { return m_anonymous; }
 
 	virtual TypePointer type() const override;
-	virtual std::shared_ptr<FunctionType> functionType(bool /*_internal*/) const override;
+	virtual FunctionTypePointer functionType(bool /*_internal*/) const override;
 
 	virtual EventDefinitionAnnotation& annotation() const override;