diff --git a/src/Moneybird.Net/Extensions/JsonElementExtensions.cs b/src/Moneybird.Net/Extensions/JsonElementExtensions.cs
new file mode 100644
index 0000000..e3858c0
--- /dev/null
+++ b/src/Moneybird.Net/Extensions/JsonElementExtensions.cs
@@ -0,0 +1,24 @@
+using System.Linq;
+using System.Text.Json;
+
+namespace Moneybird.Net.Extensions
+{
+    internal static class JsonElementExtensions
+    {
+        /// <summary>
+        /// Get the error message from the error JsonElement.
+        /// </summary>
+        /// <param name="jsonElement">The JsonElement from the error response content.</param>
+        /// <returns>The error message as a string.</returns>
+        public static string GetErrorMessage(this JsonElement jsonElement) {
+            var error = jsonElement.GetProperty("error");
+            var message = error.ValueKind switch
+            {
+                JsonValueKind.Object => string.Join(", ", error.EnumerateObject().SelectMany(prop => prop.Value.EnumerateArray().Select(val => $"{prop.Name}: {val.GetString()}")).ToList()),
+                _ => error.GetString(),
+            };
+
+            return message;
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/Moneybird.Net/Http/RequesterBase.cs b/src/Moneybird.Net/Http/RequesterBase.cs
index 2ce095e..e6edb25 100644
--- a/src/Moneybird.Net/Http/RequesterBase.cs
+++ b/src/Moneybird.Net/Http/RequesterBase.cs
@@ -1,14 +1,17 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Net;
 using System.Net.Http;
 using System.Net.Http.Headers;
 using System.Text.Json;
 using System.Threading.Tasks;
+using Moneybird.Net.Extensions;
 
 namespace Moneybird.Net.Http
 {
+    [ExcludeFromCodeCoverage]
     public abstract class RequesterBase : IDisposable
     {
         private readonly HttpClient _httpClient;
@@ -51,15 +54,15 @@ protected HttpRequestMessage ConstructRequest(string host, string relativeUrl, s
             
             return requestMessage;
         }
-        
-        protected string GetQueryString(List<string> queryParameters)
+
+        private string GetQueryString(List<string> queryParameters)
         {
             return queryParameters
                 .Where(param => !string.IsNullOrWhiteSpace(param))
-                .Aggregate(string.Empty, (current, param) => current + ("&" + param));
+                .Aggregate(string.Empty, (current, param) => current + "&" + param);
         }
-        
-        protected void HandleRequestFailure(HttpResponseMessage response)
+
+        private void HandleRequestFailure(HttpResponseMessage response)
         {
             try
             {
@@ -70,8 +73,8 @@ protected void HandleRequestFailure(HttpResponseMessage response)
                     try
                     {
                         var json = response.Content.ReadAsStringAsync().Result;
-                        var obj = JsonDocument.Parse(json);
-                        message = obj.RootElement[0].GetProperty("error").GetString();
+                        var jsonDocument = JsonDocument.Parse(json);
+                        message = jsonDocument.RootElement.GetErrorMessage();
                     }
                     catch {
                         message = response.StatusCode.ToString();
diff --git a/tests/Moneybird.Net.Tests/Extensions/JsonElementExtensionsTests.cs b/tests/Moneybird.Net.Tests/Extensions/JsonElementExtensionsTests.cs
new file mode 100644
index 0000000..cd52ace
--- /dev/null
+++ b/tests/Moneybird.Net.Tests/Extensions/JsonElementExtensionsTests.cs
@@ -0,0 +1,48 @@
+using System.Text.Json;
+using Moneybird.Net.Extensions;
+using Xunit;
+
+namespace Moneybird.Net.Tests.Extensions;
+
+public class JsonElementExtensionsTests
+{
+    [Fact]
+    public void GetErrorMessage_FromStringErrorJsonElement_Returns_CorrectString()
+    {
+        const string json = "{\"error\":\"Contact is required\",\"symbolic\":{\"contact\":\"required\"}}";
+        var jsonElement = JsonDocument.Parse(json);
+        var actualValue = jsonElement.RootElement.GetErrorMessage();
+        
+        Assert.Equal("Contact is required", actualValue);
+    }
+    
+    [Fact]
+    public void GetErrorMessage_FromObjectErrorJsonElement_Returns_CorrectString()
+    {
+        const string json = "{\"error\":{\"delivery_method\":[\"The sender address must contain an email address\"]}}";
+        var jsonElement = JsonDocument.Parse(json);
+        var actualValue = jsonElement.RootElement.GetErrorMessage();
+        
+        Assert.Equal("delivery_method: The sender address must contain an email address", actualValue);
+    }
+    
+    [Fact]
+    public void GetErrorMessage_FromObjectsErrorJsonElement_Returns_CorrectString()
+    {
+        const string json = "{\"error\":{\"firstname\":[\"is required\"],\"lastname\":[\"is required\"],\"company_name\":[\"is required\"]}}";
+        var jsonElement = JsonDocument.Parse(json);
+        var actualValue = jsonElement.RootElement.GetErrorMessage();
+        
+        Assert.Equal("firstname: is required, lastname: is required, company_name: is required", actualValue);
+    }
+    
+    [Fact]
+    public void GetErrorMessage_FromObjectMultipleErrorsJsonElement_Returns_CorrectString()
+    {
+        const string json = "{\"error\":{\"delivery_method\":[\"The delivery method is required\",\"The sender address must contain an email address\"]}}";
+        var jsonElement = JsonDocument.Parse(json);
+        var actualValue = jsonElement.RootElement.GetErrorMessage();
+        
+        Assert.Equal("delivery_method: The delivery method is required, delivery_method: The sender address must contain an email address", actualValue);
+    }
+}
\ No newline at end of file