diff --git a/Sources/SpyableMacro/Factories/ReceivedArgumentsFactory.swift b/Sources/SpyableMacro/Factories/ReceivedArgumentsFactory.swift
index 3a43f6b..9ee89b4 100644
--- a/Sources/SpyableMacro/Factories/ReceivedArgumentsFactory.swift
+++ b/Sources/SpyableMacro/Factories/ReceivedArgumentsFactory.swift
@@ -72,6 +72,15 @@ struct ReceivedArgumentsFactory {
           ),
           questionMark: .postfixQuestionMarkToken()
         )
+      } else if onlyParameterType.is(SomeOrAnyTypeSyntax.self) {
+        variableType = OptionalTypeSyntax(
+          wrappedType: TupleTypeSyntax(
+            elements: TupleTypeElementListSyntax {
+              TupleTypeElementSyntax(type: onlyParameterType)
+            }
+          ),
+          questionMark: .postfixQuestionMarkToken()
+        )
       } else {
         variableType = OptionalTypeSyntax(
           wrappedType: onlyParameterType,
diff --git a/Tests/SpyableMacroTests/Factories/UT_ReceivedArgumentsFactory.swift b/Tests/SpyableMacroTests/Factories/UT_ReceivedArgumentsFactory.swift
index 4189d55..949e530 100644
--- a/Tests/SpyableMacroTests/Factories/UT_ReceivedArgumentsFactory.swift
+++ b/Tests/SpyableMacroTests/Factories/UT_ReceivedArgumentsFactory.swift
@@ -23,6 +23,14 @@ final class UT_ReceivedArgumentsFactory: XCTestCase {
     )
   }
 
+  func testVariableDeclarationSingleExistentialTypeArgument() throws {
+    try assertProtocolFunction(
+      withFunctionDeclaration: "func foo(bar: any BarProtocol)",
+      prefixForVariable: "_prefix_",
+      expectingVariableDeclaration: "var _prefix_ReceivedBar: (any BarProtocol)?"
+    )
+  }
+
   func testVariableDeclarationSingleArgumentDoubleParameterName() throws {
     try assertProtocolFunction(
       withFunctionDeclaration: "func foo(firstName secondName: (String, Int))",
diff --git a/Tests/SpyableMacroTests/Factories/UT_SpyFactory.swift b/Tests/SpyableMacroTests/Factories/UT_SpyFactory.swift
index 4a6ffb4..6845ca9 100644
--- a/Tests/SpyableMacroTests/Factories/UT_SpyFactory.swift
+++ b/Tests/SpyableMacroTests/Factories/UT_SpyFactory.swift
@@ -65,6 +65,33 @@ final class UT_SpyFactory: XCTestCase {
     )
   }
 
+  func testDeclarationExistentialTypeArguments() throws {
+    try assertProtocol(
+      withDeclaration: """
+        protocol ViewModelProtocol {
+            func foo(model: any ModelProtocol)
+        }
+        """,
+      expectingClassDeclaration: """
+        class ViewModelProtocolSpy: ViewModelProtocol {
+            var fooModelCallsCount = 0
+            var fooModelCalled: Bool {
+                return fooModelCallsCount > 0
+            }
+            var fooModelReceivedModel: (any ModelProtocol)?
+            var fooModelReceivedInvocations: [any ModelProtocol] = []
+            var fooModelClosure: ((any ModelProtocol) -> Void)?
+            func foo(model: any ModelProtocol) {
+                fooModelCallsCount += 1
+                fooModelReceivedModel = (model)
+                fooModelReceivedInvocations.append((model))
+                fooModelClosure?(model)
+            }
+        }
+        """
+    )
+  }
+
   func testDeclarationEscapingAutoClosureArgument() throws {
     try assertProtocol(
       withDeclaration: """