diff --git a/src/RobinTTY.NordigenApiClient.Tests/LiveApi/Endpoints/AgreementsEndpointTests.cs b/src/RobinTTY.NordigenApiClient.Tests/LiveApi/Endpoints/AgreementsEndpointTests.cs
index e52d802..ec8f187 100644
--- a/src/RobinTTY.NordigenApiClient.Tests/LiveApi/Endpoints/AgreementsEndpointTests.cs
+++ b/src/RobinTTY.NordigenApiClient.Tests/LiveApi/Endpoints/AgreementsEndpointTests.cs
@@ -14,11 +14,12 @@ public void Setup()
         _apiClient = TestHelpers.GetConfiguredClient();
     }
 
+    #region RequestsWithSuccessfulResponse
+
     /// <summary>
     /// Tests the paging mechanism of retrieving end user agreements.
     /// Creates 3 agreements, retrieves them using 3 <see cref="ResponsePage{T}" />s and deletes the agreements after.
     /// </summary>
-    /// <returns></returns>
     [Test]
     public async Task GetAgreementsPaged()
     {
@@ -78,7 +79,6 @@ public async Task GetAgreementsPaged()
     /// <summary>
     /// Tests the retrieval of one agreement via <see cref="Guid" /> and string id.
     /// </summary>
-    /// <returns></returns>
     [Test]
     public async Task GetAgreement()
     {
@@ -104,7 +104,6 @@ public async Task GetAgreement()
     /// <summary>
     /// Tests the retrieval of an agreement with an invalid guid.
     /// </summary>
-    /// <returns></returns>
     [Test]
     public async Task GetAgreementWithInvalidGuid()
     {
@@ -118,7 +117,6 @@ public async Task GetAgreementWithInvalidGuid()
     /// <summary>
     /// Tests the creation and deletion of an end user agreement.
     /// </summary>
-    /// <returns></returns>
     [Test]
     public async Task CreateAcceptAndDeleteAgreement()
     {
@@ -153,42 +151,70 @@ public async Task CreateAcceptAndDeleteAgreement()
         Assert.That(deletionResponse.Result!.Summary, Is.EqualTo("End User Agreement deleted"));
     }
 
+    #endregion
+
+    #region RequestsWithErrors
+
+    /// <summary>
+    /// Tests the retrieving of an end user agreement with an invalid institution id.
+    /// </summary>
+    [Test]
+    public async Task GetAgreementWithInvalidInstitutionId()
+    {
+        var agreement = new CreateAgreementRequest(90, 90,
+            ["balances", "details", "transactions"], "invalid_institution");
+
+        var response = await _apiClient.AgreementsEndpoint.CreateAgreement(agreement);
+
+        AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(response, HttpStatusCode.BadRequest);
+        Assert.Multiple(() =>
+        {
+            Assert.That(response.Error!.InstitutionIdError, Is.Not.Null);
+            Assert.That(response.Error!.InstitutionIdError!.Summary,
+                Is.EqualTo("Unknown Institution ID invalid_institution"));
+            Assert.That(response.Error!.InstitutionIdError!.Detail,
+                Is.EqualTo("Get Institution IDs from /institutions/?country={$COUNTRY_CODE}"));
+        });
+    }
+
     /// <summary>
     /// Tests the creation of an end user agreement with an invalid institution id.
     /// </summary>
-    /// <returns></returns>
     [Test]
     public async Task CreateAgreementWithInvalidInstitutionId()
     {
-        var agreement = new CreateAgreementRequest(90, 90, ["balances", "details", "transactions"],
-            "SANDBOXFINANCE_SFIN000");
+        var agreement = new CreateAgreementRequest(90, 90,
+            ["balances", "details", "transactions"], "invalid_institution");
+
         var response = await _apiClient.AgreementsEndpoint.CreateAgreement(agreement);
-        AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(response, HttpStatusCode.BadRequest);
 
-        var result = response.Error!;
-        Assert.That(result.InstitutionIdError, Is.Not.Null);
-        Assert.That(result.InstitutionIdError!.Detail,
-            Is.EqualTo("Get Institution IDs from /institutions/?country={$COUNTRY_CODE}"));
+        AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(response, HttpStatusCode.BadRequest);
+        Assert.Multiple(() =>
+        {
+            Assert.That(response.Error!.InstitutionIdError, Is.Not.Null);
+            Assert.That(response.Error!.InstitutionIdError!.Summary,
+                Is.EqualTo("Unknown Institution ID invalid_institution"));
+            Assert.That(response.Error!.InstitutionIdError!.Detail,
+                Is.EqualTo("Get Institution IDs from /institutions/?country={$COUNTRY_CODE}"));
+        });
     }
 
     /// <summary>
     /// Tests the creation of an end user agreement with invalid parameters.
     /// </summary>
-    /// <returns></returns>
     [Test]
     public async Task CreateAgreementWithInvalidParams()
     {
         var agreement = new CreateAgreementRequest(200, 200,
             ["balances", "details", "transactions", "invalid", "invalid2"], "SANDBOXFINANCE_SFIN0000");
-        var response = await _apiClient.AgreementsEndpoint.CreateAgreement(agreement);
-        AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(response, HttpStatusCode.BadRequest);
 
+        var response = await _apiClient.AgreementsEndpoint.CreateAgreement(agreement);
         var result = response.Error!;
+
         Assert.Multiple(() =>
         {
-            Assert.That(
-                new[] {result.InstitutionIdError, result.AgreementError},
-                Has.All.Null);
+            AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(response, HttpStatusCode.BadRequest);
+            Assert.That(new[] {result.InstitutionIdError, result.AgreementError}, Has.All.Null);
             Assert.That(result.AccessScopeError!.Detail,
                 Is.EqualTo("Choose one or several from ['balances', 'details', 'transactions']"));
             Assert.That(result.AccessValidForDaysError!.Detail,
@@ -198,4 +224,25 @@ public async Task CreateAgreementWithInvalidParams()
                     "max_historical_days must be > 0 and <= SANDBOXFINANCE_SFIN0000 transaction_total_days (90)"));
         });
     }
+
+    [Test]
+    public async Task CreateAgreementWithInvalidParamsAtPolishInstitution()
+    {
+        var agreement = new CreateAgreementRequest(90, 90,
+            ["balances", "transactions"], "PKO_BPKOPLPW");
+
+        var response = await _apiClient.AgreementsEndpoint.CreateAgreement(agreement);
+        var result = response.Error!;
+
+        Assert.Multiple(() =>
+        {
+            AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(response, HttpStatusCode.BadRequest);
+            Assert.That(new[] {result.InstitutionIdError, result.AgreementError}, Has.All.Null);
+            Assert.That(result.Detail,
+                Is.EqualTo("For this institution the following scopes are required together: ['details', 'balances']"));
+            Assert.That(result.Summary, Is.EqualTo("Institution access scope dependencies error"));
+        });
+    }
+
+    #endregion
 }
diff --git a/src/RobinTTY.NordigenApiClient/JsonConverters/SingleOrArrayConverter.cs b/src/RobinTTY.NordigenApiClient/JsonConverters/SingleOrArrayConverter.cs
index 097c306..b514e84 100644
--- a/src/RobinTTY.NordigenApiClient/JsonConverters/SingleOrArrayConverter.cs
+++ b/src/RobinTTY.NordigenApiClient/JsonConverters/SingleOrArrayConverter.cs
@@ -6,7 +6,7 @@ namespace RobinTTY.NordigenApiClient.JsonConverters;
 internal class SingleOrArrayConverter<TEnumerable, TItem> : JsonConverter<TEnumerable>
     where TEnumerable : IEnumerable<TItem>
 {
-    public override TEnumerable? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    public override TEnumerable Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
     {
         switch (reader.TokenType)
         {
@@ -20,6 +20,7 @@ internal class SingleOrArrayConverter<TEnumerable, TItem> : JsonConverter<TEnume
                     var listItem = JsonSerializer.Deserialize<TItem>(ref reader, options);
                     if (listItem != null) list.Add(listItem);
                 }
+
                 return (TEnumerable) (IEnumerable<TItem>) list;
             default:
                 var item = JsonSerializer.Deserialize<TItem>(ref reader, options);
@@ -43,4 +44,4 @@ public override void Write(Utf8JsonWriter writer, TEnumerable value, JsonSeriali
             writer.WriteEndArray();
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/RobinTTY.NordigenApiClient/JsonConverters/StringArrayMergeConverter.cs b/src/RobinTTY.NordigenApiClient/JsonConverters/StringArrayMergeConverter.cs
new file mode 100644
index 0000000..508c238
--- /dev/null
+++ b/src/RobinTTY.NordigenApiClient/JsonConverters/StringArrayMergeConverter.cs
@@ -0,0 +1,37 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using RobinTTY.NordigenApiClient.Models.Responses;
+
+namespace RobinTTY.NordigenApiClient.JsonConverters;
+
+/// <summary>
+/// For some errors the GoCardless API returns arrays for Summary/Detail properties inside the <see cref="BasicResponse"/>.
+/// I've never actually seen them contain multiple values, but this converter merges them into one string so that the
+/// <see cref="BasicResponse"/> can stay as simple as possible.
+/// </summary>
+internal class StringArrayMergeConverter : JsonConverter<string>
+{
+    public override string? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        switch (reader.TokenType)
+        {
+            case JsonTokenType.Null:
+                return null;
+            case JsonTokenType.StartArray:
+                var list = new List<string>();
+                while (reader.Read())
+                {
+                    if (reader.TokenType == JsonTokenType.EndArray) break;
+                    var listItem = JsonSerializer.Deserialize<string>(ref reader, options);
+                    if (listItem != null) list.Add(listItem);
+                }
+
+                return string.Join("; ", list);
+            default:
+                return JsonSerializer.Deserialize<string>(ref reader, options);
+        }
+    }
+
+    public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options) =>
+        JsonSerializer.Serialize(writer, value, options);
+}
diff --git a/src/RobinTTY.NordigenApiClient/Models/Responses/BasicResponse.cs b/src/RobinTTY.NordigenApiClient/Models/Responses/BasicResponse.cs
index d192839..9e2f2d0 100644
--- a/src/RobinTTY.NordigenApiClient/Models/Responses/BasicResponse.cs
+++ b/src/RobinTTY.NordigenApiClient/Models/Responses/BasicResponse.cs
@@ -1,4 +1,5 @@
 using System.Text.Json.Serialization;
+using RobinTTY.NordigenApiClient.JsonConverters;
 
 namespace RobinTTY.NordigenApiClient.Models.Responses;
 
@@ -11,12 +12,14 @@ public class BasicResponse
     /// The summary text of the response/error.
     /// </summary>
     [JsonPropertyName("summary")]
+    [JsonConverter(typeof(StringArrayMergeConverter))]
     public string? Summary { get; init; }
 
     /// <summary>
     /// The detailed description of the response/error.
     /// </summary>
     [JsonPropertyName("detail")]
+    [JsonConverter(typeof(StringArrayMergeConverter))]
     public string? Detail { get; init; }
     
     /// <summary>