diff --git a/CHANGELOG.md b/CHANGELOG.md
index b396a94e7..763cc44d1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,8 @@
 # Changelog
 
+## v8.0.9
+* [Fixed build failing when using thread-loader](https://github.com/TypeStrong/ts-loader/pull/1207) - thanks @valerio
+
 ## v8.0.8
 * [Fixed memory leak when using multiple webpack instances](https://github.com/TypeStrong/ts-loader/pull/1205) - thanks @valerio
 
diff --git a/package.json b/package.json
index e5e48f498..9c12bc5cd 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "ts-loader",
-  "version": "8.0.8",
+  "version": "8.0.9",
   "description": "TypeScript loader for webpack",
   "main": "index.js",
   "types": "dist",
diff --git a/src/instance-cache.ts b/src/instance-cache.ts
new file mode 100644
index 000000000..4ed44e9a5
--- /dev/null
+++ b/src/instance-cache.ts
@@ -0,0 +1,39 @@
+import * as webpack from 'webpack';
+import { TSInstance } from './interfaces';
+
+// Some loaders (e.g. thread-loader) will set the _compiler property to undefined.
+// We can't use undefined as a WeakMap key as it will throw an error at runtime,
+// thus we keep a dummy "marker" object to use as key in those situations.
+const marker: webpack.Compiler = {} as webpack.Compiler;
+
+// Each TypeScript instance is cached based on the webpack instance (key of the WeakMap)
+// and also the name that was generated or passed via the options (string key of the
+// internal Map)
+const cache: WeakMap<webpack.Compiler, Map<string, TSInstance>> = new WeakMap();
+
+export function getTSInstanceFromCache(
+  key: webpack.Compiler,
+  name: string
+): TSInstance | undefined {
+  const compiler = key ?? marker;
+
+  let instances = cache.get(compiler);
+  if (!instances) {
+    instances = new Map();
+    cache.set(compiler, instances);
+  }
+
+  return instances.get(name);
+}
+
+export function setTSInstanceInCache(
+  key: webpack.Compiler,
+  name: string,
+  instance: TSInstance
+) {
+  const compiler = key ?? marker;
+
+  const instances = cache.get(compiler) ?? new Map<string, TSInstance>();
+  instances.set(name, instance);
+  cache.set(compiler, instances);
+}
diff --git a/src/instances.ts b/src/instances.ts
index 69008844e..f7496132a 100644
--- a/src/instances.ts
+++ b/src/instances.ts
@@ -8,6 +8,7 @@ import { makeAfterCompile } from './after-compile';
 import { getCompiler, getCompilerOptions } from './compilerSetup';
 import { getConfigFile, getConfigParseResult } from './config';
 import { dtsDtsxOrDtsDtsxMapRegex, EOL, tsTsxRegex } from './constants';
+import { getTSInstanceFromCache, setTSInstanceInCache } from './instance-cache';
 import {
   FilePathKey,
   LoaderOptions,
@@ -33,22 +34,8 @@ import {
 } from './utils';
 import { makeWatchRun } from './watch-run';
 
-// Each TypeScript instance is based on the webpack instance (key of the WeakMap)
-// and also the name that was generated or passed via the options (string key of the
-// internal Map)
-const instanceCache = new WeakMap<webpack.Compiler, Map<string, TSInstance>>();
 const instancesBySolutionBuilderConfigs = new Map<FilePathKey, TSInstance>();
 
-function addTSInstanceToCache(
-  key: webpack.Compiler,
-  instanceName: string,
-  instance: TSInstance
-) {
-  const instances = instanceCache.get(key) ?? new Map<string, TSInstance>();
-  instances.set(instanceName, instance);
-  instanceCache.set(key, instances);
-}
-
 /**
  * The loader is executed once for each file seen by webpack. However, we need to keep
  * a persistent instance of TypeScript that contains all of the files in the program
@@ -60,13 +47,10 @@ export function getTypeScriptInstance(
   loaderOptions: LoaderOptions,
   loader: webpack.loader.LoaderContext
 ): { instance?: TSInstance; error?: WebpackError } {
-  let instances = instanceCache.get(loader._compiler);
-  if (!instances) {
-    instances = new Map();
-    instanceCache.set(loader._compiler, instances);
-  }
-
-  const existing = instances.get(loaderOptions.instance);
+  const existing = getTSInstanceFromCache(
+    loader._compiler,
+    loaderOptions.instance
+  );
   if (existing) {
     if (!existing.initialSetupPending) {
       ensureProgram(existing);
@@ -160,7 +144,7 @@ function successfulTypeScriptInstance(
     const existing = getExistingSolutionBuilderHost(configFileKey);
     if (existing) {
       // Reuse the instance if config file for project references is shared.
-      addTSInstanceToCache(loader._compiler, loaderOptions.instance, existing);
+      setTSInstanceInCache(loader._compiler, loaderOptions.instance, existing);
       return { instance: existing };
     }
   }
@@ -246,7 +230,7 @@ function successfulTypeScriptInstance(
       filePathKeyMapper,
     };
 
-    addTSInstanceToCache(
+    setTSInstanceInCache(
       loader._compiler,
       loaderOptions.instance,
       transpileInstance
@@ -303,7 +287,7 @@ function successfulTypeScriptInstance(
     filePathKeyMapper,
   };
 
-  addTSInstanceToCache(loader._compiler, loaderOptions.instance, instance);
+  setTSInstanceInCache(loader._compiler, loaderOptions.instance, instance);
   return { instance };
 }