diff --git a/.github/workflows/build.yaml b/.github/workflows/build.yaml
index 5cee03c..55ac198 100644
--- a/.github/workflows/build.yaml
+++ b/.github/workflows/build.yaml
@@ -32,6 +32,7 @@ jobs:
             ${{ runner.OS }}-node-
             ${{ runner.OS }}-
       - run: npm ci
+      - run: npm run test-typescript
       - run: npm test
       - name: Coveralls Parallel
         uses: coverallsapp/github-action@master
diff --git a/apollo4.js b/apollo4.js
index c1905d1..9d41daf 100644
--- a/apollo4.js
+++ b/apollo4.js
@@ -3,9 +3,9 @@ const {
   buildSchema,
   GraphQLError
 } = require('graphql')
-const { validateQuery } = require('./index')
 const { constraintDirectiveTypeDefs } = require('./lib/type-defs')
 const { gql } = require('graphql-tag')
+const { validateQuery } = require('./lib/validate-query')
 
 let currentSchema
 
diff --git a/index.js b/index.js
index a702b1c..82d8312 100644
--- a/index.js
+++ b/index.js
@@ -1,15 +1,13 @@
 const {
   GraphQLNonNull,
   GraphQLList,
-  TypeInfo,
-  ValidationContext,
-  visit,
-  visitWithTypeInfo,
   separateOperations,
   GraphQLError,
   getDirectiveValues
 } = require('graphql')
 const QueryValidationVisitor = require('./lib/query-validation-visitor.js')
+const { validateQuery } = require('./lib/validate-query')
+
 const { getDirective, mapSchema, MapperKind } = require('@graphql-tools/utils')
 const { getConstraintTypeObject, getScalarType } = require('./lib/type-utils')
 const { constraintDirectiveTypeDefs, constraintDirectiveTypeDefsObj } = require('./lib/type-defs')
@@ -173,28 +171,6 @@ function constraintDirectiveDocumentation (options) {
     })
 }
 
-function validateQuery (schema, query, variables, operationName, pluginOptions = {}) {
-  const typeInfo = new TypeInfo(schema)
-
-  const errors = []
-  const context = new ValidationContext(
-    schema,
-    query,
-    typeInfo,
-    (error) => errors.push(error)
-  )
-
-  const visitor = new QueryValidationVisitor(context, {
-    variables,
-    operationName,
-    pluginOptions
-  })
-
-  visit(query, visitWithTypeInfo(typeInfo, visitor))
-
-  return errors
-}
-
 function createApolloQueryValidationPlugin ({ schema }, options = {}) {
   return {
     async requestDidStart () {
diff --git a/lib/error.js b/lib/error.js
index 1a1a78a..d10e845 100644
--- a/lib/error.js
+++ b/lib/error.js
@@ -1,4 +1,6 @@
 module.exports = class ConstraintDirectiveError extends Error {
+  ;
+
   constructor (fieldName, message, context) {
     super(message)
     this.name = this.constructor.name
@@ -7,5 +9,6 @@ module.exports = class ConstraintDirectiveError extends Error {
     this.code = 'ERR_GRAPHQL_CONSTRAINT_VALIDATION'
     this.fieldName = fieldName
     this.context = context
+    this.originalError = undefined
   }
 }
diff --git a/lib/type-utils.js b/lib/type-utils.js
index f1053b8..64f0abc 100644
--- a/lib/type-utils.js
+++ b/lib/type-utils.js
@@ -48,7 +48,7 @@ function getScalarType (fieldConfig) {
   } else if (isNonNullType(fieldConfig) && isScalarType(fieldConfig.ofType)) {
     return { scalarType: fieldConfig.ofType, scalarNotNull: true }
   } else if (isNonNullType(fieldConfig)) {
-    return { ...getScalarType(fieldConfig.ofType.ofType), list: true, listNotNull: true }
+    return { ...getScalarType(fieldConfig.ofType), list: true, listNotNull: true }
   } else {
     throw new Error(`Not a valid scalar type: ${fieldConfig.toString()}`)
   }
diff --git a/lib/validate-query.js b/lib/validate-query.js
new file mode 100644
index 0000000..a227c71
--- /dev/null
+++ b/lib/validate-query.js
@@ -0,0 +1,31 @@
+const {
+  TypeInfo,
+  ValidationContext,
+  visit,
+  visitWithTypeInfo
+} = require('graphql')
+const QueryValidationVisitor = require('./query-validation-visitor.js')
+
+function validateQuery (schema, query, variables, operationName, pluginOptions = {}) {
+  const typeInfo = new TypeInfo(schema)
+
+  const errors = []
+  const context = new ValidationContext(
+    schema,
+    query,
+    typeInfo,
+    (error) => errors.push(error)
+  )
+
+  const visitor = new QueryValidationVisitor(context, {
+    variables,
+    operationName,
+    pluginOptions
+  })
+
+  visit(query, visitWithTypeInfo(typeInfo, visitor))
+
+  return errors
+}
+
+module.exports = { validateQuery }
diff --git a/package-lock.json b/package-lock.json
index 361b2bf..38138e2 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -23,7 +23,8 @@
         "mocha": "10.7.3",
         "nyc": "15.1.0",
         "standard": "16.0.4",
-        "supertest": "6.3.4"
+        "supertest": "6.3.4",
+        "typescript": "^5.5.3"
       },
       "engines": {
         "node": ">=14.0.0"
@@ -7101,6 +7102,19 @@
         "is-typedarray": "^1.0.0"
       }
     },
+    "node_modules/typescript": {
+      "version": "5.5.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.5.3.tgz",
+      "integrity": "sha512-/hreyEujaB0w76zKo6717l3L0o/qEUtRgdvUBvlkhoWeOVMjMuHNHk0BRBzikzuGDqNmPQbg5ifMEqsHLiIUcQ==",
+      "dev": true,
+      "bin": {
+        "tsc": "bin/tsc",
+        "tsserver": "bin/tsserver"
+      },
+      "engines": {
+        "node": ">=14.17"
+      }
+    },
     "node_modules/unbox-primitive": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/unbox-primitive/-/unbox-primitive-1.0.1.tgz",
@@ -12945,6 +12959,12 @@
         "is-typedarray": "^1.0.0"
       }
     },
+    "typescript": {
+      "version": "5.5.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.5.3.tgz",
+      "integrity": "sha512-/hreyEujaB0w76zKo6717l3L0o/qEUtRgdvUBvlkhoWeOVMjMuHNHk0BRBzikzuGDqNmPQbg5ifMEqsHLiIUcQ==",
+      "dev": true
+    },
     "unbox-primitive": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/unbox-primitive/-/unbox-primitive-1.0.1.tgz",
diff --git a/package.json b/package.json
index 526bf52..0bdc903 100644
--- a/package.json
+++ b/package.json
@@ -11,6 +11,7 @@
     "test-apollo-plugin": "standard && nyc --reporter=html --reporter=text --reporter=lcov mocha test/**/testsuite-apollo-plugin.js",
     "test-apollo4-plugin": "standard && nyc --reporter=html --reporter=text --reporter=lcov mocha test/**/testsuite-apollo4-plugin.js",
     "test-envelop-plugin": "standard && nyc --reporter=html --reporter=text --reporter=lcov mocha test/**/testsuite-envelop-plugin.js",
+    "test-typescript": "tsc --project tsconfig.apollo4.json && tsc --project tsconfig.apollo.json",
     "test-validation-rule-express-graphql": "standard && nyc --reporter=html --reporter=text --reporter=lcov mocha test/**/testsuite-validation-rule-express-graphql.js"
   },
   "author": "James Mortemore (jamesmortemore@gmail.com)",
@@ -34,8 +35,9 @@
     "constraint"
   ],
   "devDependencies": {
-    "apollo-server-express": "3.13.0",
     "@apollo/server": "4.9.5",
+    "@graphql-yoga/node": "2.13.13",
+    "apollo-server-express": "3.13.0",
     "coveralls": "3.1.1",
     "express": "4.21.0",
     "graphql": "16.6.0",
@@ -43,7 +45,7 @@
     "nyc": "15.1.0",
     "standard": "16.0.4",
     "supertest": "6.3.4",
-    "@graphql-yoga/node": "2.13.13"
+    "typescript": "^5.5.3"
   },
   "dependencies": {
     "@graphql-tools/schema": "^9.0.0",
diff --git a/test/typescript/apollo.ts b/test/typescript/apollo.ts
new file mode 100644
index 0000000..1f9b5fe
--- /dev/null
+++ b/test/typescript/apollo.ts
@@ -0,0 +1 @@
+import { constraintDirective, constraintDirectiveTypeDefs } from 'graphql-constraint-directive';
diff --git a/test/typescript/apollo4.ts b/test/typescript/apollo4.ts
new file mode 100644
index 0000000..60f54b1
--- /dev/null
+++ b/test/typescript/apollo4.ts
@@ -0,0 +1,3 @@
+import { constraintDirectiveTypeDefs, createApollo4QueryValidationPlugin } from 'graphql-constraint-directive/apollo4';
+
+export { constraintDirectiveTypeDefs, createApollo4QueryValidationPlugin };
\ No newline at end of file
diff --git a/test/typescript/error.ts b/test/typescript/error.ts
new file mode 100644
index 0000000..abf7e4d
--- /dev/null
+++ b/test/typescript/error.ts
@@ -0,0 +1,3 @@
+const errorFn: () => string = {
+
+}
\ No newline at end of file
diff --git a/tsconfig.apollo.json b/tsconfig.apollo.json
new file mode 100644
index 0000000..f2f8b79
--- /dev/null
+++ b/tsconfig.apollo.json
@@ -0,0 +1,32 @@
+{
+    "compileOnSave": false,
+    "compilerOptions": {
+        "baseUrl": ".",
+        "rootDir": ".",
+        "declaration": false,
+        "moduleResolution": "node",
+        "noEmit": true,
+        "importHelpers": true,
+        "target": "es2015",
+        "module": "esnext",
+        "lib": [
+            "es2020",
+        ],
+        "checkJs": true,
+        "skipLibCheck": true,
+        "skipDefaultLibCheck": true,
+        "paths": {
+            "graphql-constraint-directive": [
+                "index.js",
+                "index.d.ts",
+            ]
+        }
+    },
+    "include": [
+        "./test/typescript/apollo.ts"
+    ],
+    "exclude": [
+        "node_modules",
+        "tmp"
+    ]
+}
\ No newline at end of file
diff --git a/tsconfig.apollo4.json b/tsconfig.apollo4.json
new file mode 100644
index 0000000..e464e21
--- /dev/null
+++ b/tsconfig.apollo4.json
@@ -0,0 +1,35 @@
+{
+    "compileOnSave": false,
+    "compilerOptions": {
+        "baseUrl": ".",
+        "rootDir": ".",
+        "declaration": false,
+        "moduleResolution": "node",
+        "noEmit": true,
+        "importHelpers": true,
+        "target": "es2015",
+        "module": "esnext",
+        "lib": [
+            "es2020",
+        ],
+        "checkJs": true,
+        "skipLibCheck": true,
+        "skipDefaultLibCheck": true,
+        "paths": {
+            "graphql-constraint-directive/apollo4": [
+                "apollo4.js",
+                "apollo4.d.ts",
+            ],
+            "apollo-server-errors": [
+                "./test/typescript/error.ts"
+            ]
+        }
+    },
+    "include": [
+        "./test/typescript/apollo4.ts"
+    ],
+    "exclude": [
+        "node_modules",
+        "tmp"
+    ]
+}
\ No newline at end of file