diff --git a/src/nunit.analyzers.tests/WithinUsage/WithinUsageAnalyzerTests.cs b/src/nunit.analyzers.tests/WithinUsage/WithinUsageAnalyzerTests.cs
index 6bbb1a40..9f74f542 100644
--- a/src/nunit.analyzers.tests/WithinUsage/WithinUsageAnalyzerTests.cs
+++ b/src/nunit.analyzers.tests/WithinUsage/WithinUsageAnalyzerTests.cs
@@ -273,6 +273,31 @@ public void AnalyzeOnNonGenericCollection()
             RoslynAssert.Valid(analyzer, testCode);
         }
 
+        [Test]
+        public void AnalyzeNonGenericSubclassOfGenericIEnumerable()
+        {
+            string testCode = TestUtility.WrapClassInNamespaceAndAddUsing(@"
+                 [TestFixture]
+                 public class TestClass
+                 {
+                     public void TestMethod()
+                     {
+                         Assert.That(new DoubleCollection(1), Is.EqualTo(new DoubleCollection(2)).Within(0.001));
+                     }
+
+                     private sealed class DoubleCollection : IEnumerable<double>
+                     {
+                         public DoubleCollection(params double[] numbers) { }
+                         public IEnumerator<double> GetEnumerator() => throw new NotImplementedException();
+                         IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+                     }
+                 }",
+                additionalUsings: @"using System.Collections;
+ using System.Collections.Generic;");
+
+            RoslynAssert.Valid(analyzer, testCode);
+        }
+
         [TestCase("double")]
         [TestCase("int")]
         [TestCase("TimeSpan")]
diff --git a/src/nunit.analyzers/WithinUsage/WithinUsageAnalyzer.cs b/src/nunit.analyzers/WithinUsage/WithinUsageAnalyzer.cs
index 9519a5f5..09c2e9b3 100644
--- a/src/nunit.analyzers/WithinUsage/WithinUsageAnalyzer.cs
+++ b/src/nunit.analyzers/WithinUsage/WithinUsageAnalyzer.cs
@@ -117,7 +117,7 @@ private static bool IsTypeSupported(ITypeSymbol type, HashSet<ITypeSymbol>? chec
                 string interfaceTypeName = interfaceType.GetFullMetadataName();
                 if (interfaceTypeName.StartsWith("System.Collections.Generic.IEnumerable`", StringComparison.Ordinal))
                 {
-                    return IsTypeSupported(namedType.TypeArguments[0], checkedTypes);
+                    return IsTypeSupported(interfaceType.TypeArguments[0], checkedTypes);
                 }
 
                 if (interfaceTypeName.Equals("System.Collections.IEnumerable", StringComparison.Ordinal))