diff --git a/src/commands/generate-prep.ts b/src/commands/generate-prep.ts
index 55fc630..ed80264 100644
--- a/src/commands/generate-prep.ts
+++ b/src/commands/generate-prep.ts
@@ -6,6 +6,7 @@ import { BlobEntry } from '../blobs/entry'
 import { loadDeviceConfig } from '../config/device'
 import { enumerateFiles, extractProps, generateBuildFiles, PropResults } from '../frontend/generate'
 import { withSpinner } from '../util/cli'
+import { withTempDir } from '../util/fs'
 
 export default class GeneratePrep extends Command {
   static description = 'generate vendor parts to prepare for reference AOSP build (e.g. for collect-state)'
@@ -25,38 +26,41 @@ export default class GeneratePrep extends Command {
 
     let config = await loadDeviceConfig(configPath)
 
-    // Each step will modify this. Key = combined part path
-    let namedEntries = new Map<string, BlobEntry>()
+    // tmp may be needed for mounting/extracting stock images
+    await withTempDir(async (tmp) => {
+      // Each step will modify this. Key = combined part path
+      let namedEntries = new Map<string, BlobEntry>()
 
-    // Prepare output directories
-    let dirs = await createVendorDirs(config.device.vendor, config.device.name)
+      // Prepare output directories
+      let dirs = await createVendorDirs(config.device.vendor, config.device.name)
 
-    // 1. Diff files
-    await withSpinner('Enumerating files', (spinner) =>
-      enumerateFiles(spinner, config.filters.dep_files, null, namedEntries, null,
-        stockSrc, null))
+      // 1. Diff files
+      await withSpinner('Enumerating files', (spinner) =>
+        enumerateFiles(spinner, config.filters.dep_files, null, namedEntries, null,
+          stockSrc))
 
-    // After this point, we only need entry objects
-    let entries = Array.from(namedEntries.values())
+      // After this point, we only need entry objects
+      let entries = Array.from(namedEntries.values())
 
-    // 2. Extract
-    // Copy blobs (this has its own spinner)
-    if (config.generate.files && !skipCopy) {
-      await copyBlobs(entries, stockSrc, dirs.proprietary)
-    }
+      // 2. Extract
+      // Copy blobs (this has its own spinner)
+      if (config.generate.files && !skipCopy) {
+        await copyBlobs(entries, stockSrc, dirs.proprietary)
+      }
 
-    // 3. Props
-    let propResults: PropResults | null = null
-    if (config.generate.props) {
-      propResults = await withSpinner('Extracting properties', () =>
-        extractProps(config, null, stockSrc, null))
-      delete propResults.missingProps
-      delete propResults.fingerprint
-    }
+      // 3. Props
+      let propResults: PropResults | null = null
+      if (config.generate.props) {
+        propResults = await withSpinner('Extracting properties', () =>
+          extractProps(config, null, stockSrc))
+        delete propResults.missingProps
+        delete propResults.fingerprint
+      }
 
-    // 4. Build files
-    await withSpinner('Generating build files', () =>
-      generateBuildFiles(config, dirs, entries, [], propResults, null, null, null,
-        stockSrc, false, true))
+      // 4. Build files
+      await withSpinner('Generating build files', () =>
+        generateBuildFiles(config, dirs, entries, [], propResults, null, null, null,
+          stockSrc, false, true))
+    })
   }
 }
diff --git a/src/frontend/generate.ts b/src/frontend/generate.ts
index 4c4d29f..f3016e4 100644
--- a/src/frontend/generate.ts
+++ b/src/frontend/generate.ts
@@ -34,13 +34,13 @@ export async function enumerateFiles(
   filters: Filters,
   forceIncludeFilters: Filters | null,
   namedEntries: Map<string, BlobEntry>,
-  customState: SystemState,
+  customState: SystemState | null,
   stockSrc: string,
 ) {
   for (let partition of ALL_SYS_PARTITIONS) {
     let filesRef = await listPart(partition, stockSrc, filters)
     if (filesRef == null) continue
-    let filesNew = customState.partitionFiles[partition]
+    let filesNew = customState?.partitionFiles[partition] ?? []
     if (filesNew == undefined) continue
 
     let missingFiles = diffLists(filesNew, filesRef)
@@ -117,11 +117,11 @@ export async function flattenApexs(
 
 export async function extractProps(
   config: DeviceConfig,
-  customState: SystemState,
+  customState: SystemState | null,
   stockSrc: string,
 ) {
   let stockProps = await loadPartitionProps(stockSrc)
-  let customProps = customState.partitionProps
+  let customProps = customState?.partitionProps ?? new Map<string, Map<string, string>>()
 
   // Filters
   for (let props of stockProps.values()) {