diff --git a/Sources/GraphQL/Execution/Execute.swift b/Sources/GraphQL/Execution/Execute.swift
index b0a21f8f..dcfc3f07 100644
--- a/Sources/GraphQL/Execution/Execute.swift
+++ b/Sources/GraphQL/Execution/Execute.swift
@@ -78,7 +78,11 @@ public final class ExecutionContext: @unchecked Sendable {
     }
 
     public func append(error: GraphQLError) {
-        errors.append(error)
+        // `append` must explicitly use the DispatchQueue and the underlying storage because by
+        // default `append` uses separate unblocked get, modify, and replace steps.
+        errorsQueue.sync(flags: .barrier) {
+            self._errors.append(error)
+        }
     }
 }
 
@@ -849,7 +853,7 @@ func completeListValue(
 
     return try await withThrowingTaskGroup(of: (Int, (any Sendable)?).self) { group in
         // To preserve order, match size to result, and filter out nils at the end.
-        var results: [(any Sendable)?] = result.map { _ in nil }
+        var results = [(any Sendable)?](repeating: nil, count: result.count)
         for (index, item) in result.enumerated() {
             group.addTask {
                 // No need to modify the info object containing the path,
diff --git a/Sources/GraphQL/Type/Schema.swift b/Sources/GraphQL/Type/Schema.swift
index e0bcddbf..95a9989d 100644
--- a/Sources/GraphQL/Type/Schema.swift
+++ b/Sources/GraphQL/Type/Schema.swift
@@ -34,26 +34,9 @@ public final class GraphQLSchema: @unchecked Sendable {
     let astNode: SchemaDefinition?
     let extensionASTNodes: [SchemaExtensionDefinition]
 
-    // Used as a cache for validateSchema().
-    private var _validationErrors: [GraphQLError]?
-    private let validationErrorQueue = DispatchQueue(
-        label: "graphql.schema.validationerrors",
-        attributes: .concurrent
-    )
-    var validationErrors: [GraphQLError]? {
-        get {
-            // Reads can occur concurrently.
-            return validationErrorQueue.sync {
-                _validationErrors
-            }
-        }
-        set {
-            // Writes occur sequentially.
-            return validationErrorQueue.sync(flags: .barrier) {
-                self._validationErrors = newValue
-            }
-        }
-    }
+    /// Used as a cache for validateSchema(). Concurrent access is not protected, so assume
+    /// it can be changed at any time.
+    var validationErrors: [GraphQLError]?
 
     public let queryType: GraphQLObjectType?
     public let mutationType: GraphQLObjectType?
@@ -95,7 +78,7 @@ public final class GraphQLSchema: @unchecked Sendable {
         extensionASTNodes: [SchemaExtensionDefinition] = [],
         assumeValid: Bool = false
     ) throws {
-        _validationErrors = assumeValid ? [] : nil
+        validationErrors = assumeValid ? [] : nil
 
         self.description = description
         self.extensions = extensions