diff --git a/Sources/ApolloCodegenLib/Templates/MockObjectTemplate.swift b/Sources/ApolloCodegenLib/Templates/MockObjectTemplate.swift
index cb8d1dd56..61ff93560 100644
--- a/Sources/ApolloCodegenLib/Templates/MockObjectTemplate.swift
+++ b/Sources/ApolloCodegenLib/Templates/MockObjectTemplate.swift
@@ -76,6 +76,8 @@ struct MockObjectTemplate: TemplateRenderer {
   
   private func mockFunctionDescriptor(_ graphQLType: GraphQLType) -> String {
     switch graphQLType {
+      case .list(.nonNull(.scalar(_))):
+        return "ScalarList"
       case .list(_):
         return "List"
       case .scalar(_), .enum(_):
@@ -95,7 +97,7 @@ struct MockObjectTemplate: TemplateRenderer {
       \(if: $0.propertyName.isConflictingTestMockFieldName, """
         var \($0.propertyName): \($0.mockType)? {
           get { _data["\($0.propertyName)"] as? \($0.mockType) }
-          set { _set(newValue, for: \\.\($0.propertyName)) }
+          set { _set\(mockFunctionDescriptor($0.type))(newValue, for: \\.\($0.propertyName)) }
         }
         """)
       """ }, separator: "\n", terminator: "\n")
diff --git a/Sources/ApolloTestSupport/TestMock.swift b/Sources/ApolloTestSupport/TestMock.swift
index 656af374a..8211a3736 100644
--- a/Sources/ApolloTestSupport/TestMock.swift
+++ b/Sources/ApolloTestSupport/TestMock.swift
@@ -75,6 +75,26 @@ public class Mock<O: MockObject>: AnyMock, Hashable {
     _data[field.key.description] = value?._unsafelyConvertToMockValue()
   }
 
+  public subscript<T: AnyScalarType & Hashable>(
+    dynamicMember keyPath: KeyPath<O.MockFields, Field<Array<T>>>
+  ) -> [T]? {
+    get {
+      let field = O._mockFields[keyPath: keyPath]
+      return _data[field.key.description] as? [T]
+    }
+    set {
+      _setScalarList(newValue, for: keyPath)
+    }
+  }
+
+  public func _setScalarList<T: AnyScalarType & Hashable>(
+    _ value: [T]?,
+    for keyPath: KeyPath<O.MockFields, Field<Array<T>>>
+  ) {
+    let field = O._mockFields[keyPath: keyPath]
+    _data[field.key.description] = value?._unsafelyConvertToMockValue()
+  }
+
   public var _selectionSetMockData: JSONObject {
     _data.mapValues {
       if let mock = $0 as? AnyMock {
diff --git a/Tests/ApolloCodegenTests/CodeGeneration/Templates/MockObjectTemplateTests.swift b/Tests/ApolloCodegenTests/CodeGeneration/Templates/MockObjectTemplateTests.swift
index 182a73f00..a25db3341 100644
--- a/Tests/ApolloCodegenTests/CodeGeneration/Templates/MockObjectTemplateTests.swift
+++ b/Tests/ApolloCodegenTests/CodeGeneration/Templates/MockObjectTemplateTests.swift
@@ -462,7 +462,7 @@ class MockObjectTemplateTests: XCTestCase {
     let expected = """
       var hash: String? {
         get { _data["hash"] as? String }
-        set { _set(newValue, for: \\.hash) }
+        set { _setScalar(newValue, for: \\.hash) }
       }
 
     """
@@ -487,6 +487,7 @@ class MockObjectTemplateTests: XCTestCase {
     subject.graphqlObject.fields = [
       "string": .mock("string", type: .nonNull(.string())),
       "customScalar": .mock("customScalar", type: .nonNull(.scalar(.mock(name: "CustomScalar")))),
+      "customScalarList": .mock("customScalarList", type: .list(.nonNull(.scalar(.mock(name: "CustomScalar"))))),
       "optionalString": .mock("optionalString", type: .string()),
       "object": .mock("object", type: Cat),
       "objectList": .mock("objectList", type: .list(.nonNull(Cat))),
@@ -516,6 +517,7 @@ class MockObjectTemplateTests: XCTestCase {
     public extension Mock where O == Dog {
       convenience init(
         customScalar: TestSchema.CustomScalar? = nil,
+        customScalarList: [TestSchema.CustomScalar]? = nil,
         enumType: GraphQLEnum<TestSchema.EnumType>? = nil,
         interface: AnyMock? = nil,
         interfaceList: [AnyMock]? = nil,
@@ -534,6 +536,7 @@ class MockObjectTemplateTests: XCTestCase {
       ) {
         self.init()
         _setScalar(customScalar, for: \\.customScalar)
+        _setScalarList(customScalarList, for: \\.customScalarList)
         _setScalar(enumType, for: \\.enumType)
         _setEntity(interface, for: \\.interface)
         _setList(interfaceList, for: \\.interfaceList)
diff --git a/Tests/CodegenIntegrationTests/Tests/3120-listOfCustomScalarsNeedsSetScalarList/README.MD b/Tests/CodegenIntegrationTests/Tests/3120-listOfCustomScalarsNeedsSetScalarList/README.MD
new file mode 100644
index 000000000..6b46dc939
--- /dev/null
+++ b/Tests/CodegenIntegrationTests/Tests/3120-listOfCustomScalarsNeedsSetScalarList/README.MD
@@ -0,0 +1,10 @@
+# Overview
+
+When a field has a list type containing custom scalars, the generated initializer for mock objects should use a version of the `_set` function that is appropriate for that type. 
+Previously, the `_setList` function was used for all lists of objects, even if it would not compile since custom scalars do not conform to `GraphQLField`.
+
+## Reference Issue: https://github.com/apollographql/apollo-ios/pull/3120
+
+## Solution
+
+All properties that are lists of scalars should use the `_setScalarList` function when initialized inside mock objects.
diff --git a/Tests/CodegenIntegrationTests/Tests/3120-listOfCustomScalarsNeedsSetScalarList/operation.graphql b/Tests/CodegenIntegrationTests/Tests/3120-listOfCustomScalarsNeedsSetScalarList/operation.graphql
new file mode 100644
index 000000000..f45ed1954
--- /dev/null
+++ b/Tests/CodegenIntegrationTests/Tests/3120-listOfCustomScalarsNeedsSetScalarList/operation.graphql
@@ -0,0 +1,6 @@
+query TestMeWithMocks {
+  testMeWithMocks {
+    scalarList
+    nullableScalarList
+  }
+}
diff --git a/Tests/CodegenIntegrationTests/Tests/3120-listOfCustomScalarsNeedsSetScalarList/schema.graphqls b/Tests/CodegenIntegrationTests/Tests/3120-listOfCustomScalarsNeedsSetScalarList/schema.graphqls
new file mode 100644
index 000000000..d996243cd
--- /dev/null
+++ b/Tests/CodegenIntegrationTests/Tests/3120-listOfCustomScalarsNeedsSetScalarList/schema.graphqls
@@ -0,0 +1,10 @@
+type Query {
+  testMeWithMocks: Event!
+}
+
+type Event {
+  scalarList: [Text!]!
+  nullableScalarList: [Text]!
+}
+
+scalar Text