diff --git a/packages/cli/src/commands/build-cache/cache.ts b/packages/cli/src/commands/build-cache/cache.ts
index a6c51671f5..4dddaedc6f 100644
--- a/packages/cli/src/commands/build-cache/cache.ts
+++ b/packages/cli/src/commands/build-cache/cache.ts
@@ -96,6 +96,10 @@ export class Cache {
     for (const quotedInputPath of quotedInputPaths) {
       const output = await runPlain(`git ls-tree HEAD ${quotedInputPath}`);
       const [, , sha] = output.split(/\s+/, 3);
+      // If we can't get a tree sha it means we're outside of tracked files, so treat as dirty
+      if (!sha) {
+        return undefined;
+      }
       trees.push(sha);
     }