diff --git a/src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/SessionState/Serialization/HttpSessionStateBaseWrapper.cs b/src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/SessionState/Serialization/HttpSessionStateBaseWrapper.cs
index c06b603016..8a58fe0ef7 100644
--- a/src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/SessionState/Serialization/HttpSessionStateBaseWrapper.cs
+++ b/src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/SessionState/Serialization/HttpSessionStateBaseWrapper.cs
@@ -54,7 +54,17 @@ public bool IsAbandoned
         }
     }
 
-    public IEnumerable<string> Keys => State.Keys.Cast<string>();
+    public IEnumerable<string> Keys
+    {
+        get
+        {
+            // If the underlying storage mechanism is SessionStateItemCollection then there can be issues around the enumerator causing side effects.
+            // There are precautions in the implementation to prevent this, but we have seen an exception around this in practice (see dotnet/systemweb-adapters#556).
+            // However, there is no simple way to check if this is the case, so we'll preemptively create a copy.
+            // See https://referencesource.microsoft.com/#System.Web/State/SessionStateItemCollection.cs,435 for the warnings in the implementation.
+            return [.. State.Keys.Cast<string>()];
+        }
+    }
 
     public void Clear() => State.Clear();