diff --git a/src/commands/new.ts b/src/commands/new.ts
index 2d85e7e..2cbee53 100644
--- a/src/commands/new.ts
+++ b/src/commands/new.ts
@@ -96,6 +96,7 @@ export async function action(
     die(`Session "${branch}" already exists. Use "sandlot open ${branch}" to re-enter it.`)
   }
 
+  const branchExisted = await git.branchExists(branch, root)
   const spin = spinner("Creating worktree", branch)
   try {
     await git.createWorktree(branch, worktreeAbs, root)
diff --git a/src/git.ts b/src/git.ts
index 02ecfa1..9a11c51 100644
--- a/src/git.ts
+++ b/src/git.ts
@@ -49,16 +49,15 @@ export async function createWorktree(branch: string, worktreePath: string, cwd:
   const exists = await branchExists(branch, cwd, { fetch: true })
 
   let result
+  let switchedFromBranch = false
   if (exists === "local") {
     // If the branch is checked out in the main worktree, switch it to main first
     if (await currentBranch(cwd) === branch) {
-      await checkout(await mainBranch(cwd), cwd)
-      result = await $`git worktree add ${worktreePath} ${branch}`.cwd(cwd).nothrow().quiet()
-      if (result.exitCode !== 0) {
-        await $`git checkout ${branch}`.cwd(cwd).nothrow().quiet()
-        throw new Error(`Failed to create worktree for "${branch}": ${result.stderr.toString().trim()}`)
+      if (await isDirty(cwd)) {
+        throw new Error(`Cannot move branch "${branch}" to a worktree: the main worktree has uncommitted changes. Commit or stash them first.`)
       }
-      return
+      await checkout(await mainBranch(cwd), cwd)
+      switchedFromBranch = true
     }
     result = await $`git worktree add ${worktreePath} ${branch}`.cwd(cwd).nothrow().quiet()
   } else if (exists === "remote") {
@@ -68,6 +67,7 @@ export async function createWorktree(branch: string, worktreePath: string, cwd:
     result = await $`git worktree add -b ${branch} ${worktreePath}`.cwd(cwd).nothrow().quiet()
   }
   if (result.exitCode !== 0) {
+    if (switchedFromBranch) await checkout(branch, cwd).catch(() => {})
     throw new Error(`Failed to create worktree for "${branch}": ${result.stderr.toString().trim()}`)
   }
 }
@@ -238,6 +238,9 @@ export async function branchDiff(branch: string, main: string, cwd: string): Pro
 /** Detect the main branch name (main or master). */
 export async function mainBranch(cwd?: string): Promise<string> {
   const dir = cwd ?? "."
-  const result = await $`git -C ${dir} rev-parse --verify --quiet refs/heads/main`.nothrow().quiet()
-  return result.exitCode === 0 ? "main" : "master"
+  const main = await $`git -C ${dir} rev-parse --verify --quiet refs/heads/main`.nothrow().quiet()
+  if (main.exitCode === 0) return "main"
+  const master = await $`git -C ${dir} rev-parse --verify --quiet refs/heads/master`.nothrow().quiet()
+  if (master.exitCode === 0) return "master"
+  throw new Error("Could not detect main branch: neither \"main\" nor \"master\" exists.")
 }