diff --git a/.swiftpm/xcode/xcshareddata/xcschemes/Relax-Package.xcscheme b/.swiftpm/xcode/xcshareddata/xcschemes/Relax-Package.xcscheme
index 7b27a32..1809d9f 100644
--- a/.swiftpm/xcode/xcshareddata/xcschemes/Relax-Package.xcscheme
+++ b/.swiftpm/xcode/xcshareddata/xcschemes/Relax-Package.xcscheme
@@ -67,6 +67,16 @@
                ReferencedContainer = "container:">
             </BuildableReference>
          </TestableReference>
+         <TestableReference
+            skipped = "NO">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "RelaxMacrosTests"
+               BuildableName = "RelaxMacrosTests"
+               BlueprintName = "RelaxMacrosTests"
+               ReferencedContainer = "container:">
+            </BuildableReference>
+         </TestableReference>
       </Testables>
    </TestAction>
    <LaunchAction
diff --git a/Sources/RelaxMacros/APIServiceMacro.swift b/Sources/RelaxMacros/APIServiceMacro.swift
index 9d66e2d..c5aa9db 100644
--- a/Sources/RelaxMacros/APIServiceMacro.swift
+++ b/Sources/RelaxMacros/APIServiceMacro.swift
@@ -8,6 +8,8 @@
 import SwiftSyntax
 import SwiftSyntaxBuilder
 import SwiftSyntaxMacros
+import SwiftDiagnostics
+import Foundation
 
 public struct APIServiceMacro: ExtensionMacro {
     public static func expansion(
@@ -17,14 +19,27 @@ public struct APIServiceMacro: ExtensionMacro {
         conformingTo protocols: [TypeSyntax],
         in context: some MacroExpansionContext
     ) throws -> [ExtensionDeclSyntax] {
-        guard let arguments = node.arguments?.as(LabeledExprListSyntax.self) else {
+        guard let urlString = node
+            .arguments?.as(LabeledExprListSyntax.self)?
+            .first?.expression.as(StringLiteralExprSyntax.self)?
+            .segments
+            .first?.trimmedDescription else {
+            let error = Diagnostic(node: node, message: RelaxMacroDiagnostic.invalidBaseURL)
+            context.diagnose(error)
+            return []
+        }
+        
+        // check that the URL will be valid
+        guard URL(string: urlString) != nil else {
+            let error = Diagnostic(node: node, message: RelaxMacroDiagnostic.invalidBaseURL)
+            context.diagnose(error)
             return []
         }
         
         let decl: DeclSyntax =
         """
         extension \(type.trimmed): APIComponent {
-            static let baseURL: URL = URL(string: \(raw: arguments.trimmedDescription))!
+            static let baseURL: URL = URL(string: \"\(raw: urlString)\")!
         }
         """
         
diff --git a/Sources/RelaxMacros/RelaxMacroDiagnostic.swift b/Sources/RelaxMacros/RelaxMacroDiagnostic.swift
new file mode 100644
index 0000000..078dfef
--- /dev/null
+++ b/Sources/RelaxMacros/RelaxMacroDiagnostic.swift
@@ -0,0 +1,25 @@
+//
+//  RelaxMacroDiagnostic.swift
+//
+//
+//  Created by Thomas De Leon on 1/2/24.
+//
+
+import SwiftDiagnostics
+
+enum RelaxMacroDiagnostic: String, DiagnosticMessage {
+    case invalidBaseURL
+    
+    var severity: DiagnosticSeverity { .error }
+    
+    var message: String {
+        switch self {
+        case .invalidBaseURL:
+            "The base URL is invalid."
+        }
+    }
+    
+    var diagnosticID: MessageID {
+        MessageID(domain: "RelaxMacros", id: rawValue)
+    }
+}
diff --git a/Tests/RelaxMacrosTests/MacroTests.swift b/Tests/RelaxMacrosTests/MacroTests.swift
index b89f331..a281765 100644
--- a/Tests/RelaxMacrosTests/MacroTests.swift
+++ b/Tests/RelaxMacrosTests/MacroTests.swift
@@ -8,6 +8,7 @@
 import SwiftSyntaxMacros
 import SwiftSyntaxMacrosTestSupport
 import XCTest
+@testable import RelaxMacros
 
 #if canImport(RelaxMacros)
 import RelaxMacros
@@ -39,4 +40,24 @@ final class MacroTests: XCTestCase {
         throw XCTSkip("macros are only supported when running tests for the host platform")
         #endif
     }
+    
+    func testAPIServiceInvalidBaseURL() throws {
+        #if canImport(RelaxMacros)
+        assertMacroExpansion(
+            """
+            @APIService("https:// .com")
+            enum TestService {
+            }
+            """,
+            expandedSource: """
+            enum TestService {
+            }
+            """,
+            diagnostics: [.init(message: RelaxMacroDiagnostic.invalidBaseURL.message, line: 1, column: 1)],
+            macros: testMacros
+        )
+        #else
+        throw XCTSkip("macros are only supported when running tests for the host platform")
+        #endif
+    }
 }