diff --git a/cmd/pulls.go b/cmd/pulls.go
index b89011e..db789db 100644
--- a/cmd/pulls.go
+++ b/cmd/pulls.go
@@ -12,6 +12,7 @@ import (
 	"code.gitea.io/tea/modules/interact"
 	"code.gitea.io/tea/modules/print"
 	"code.gitea.io/tea/modules/utils"
+	"code.gitea.io/tea/modules/workaround"
 
 	"code.gitea.io/sdk/gitea"
 	"github.com/urfave/cli/v2"
@@ -65,6 +66,9 @@ func runPullDetail(cmd *cli.Context, index string) error {
 	if err != nil {
 		return err
 	}
+	if err := workaround.FixPullHeadSha(client, pr); err != nil {
+		return err
+	}
 
 	reviews, _, err := client.ListPullReviews(ctx.Owner, ctx.Repo, idx, gitea.ListPullReviewsOptions{})
 	if err != nil {
diff --git a/modules/task/pull_checkout.go b/modules/task/pull_checkout.go
index 573b11d..6140ea0 100644
--- a/modules/task/pull_checkout.go
+++ b/modules/task/pull_checkout.go
@@ -9,6 +9,7 @@ import (
 
 	"code.gitea.io/tea/modules/config"
 	local_git "code.gitea.io/tea/modules/git"
+	"code.gitea.io/tea/modules/workaround"
 
 	"github.com/go-git/go-git/v5"
 	git_plumbing "github.com/go-git/go-git/v5/plumbing"
@@ -28,6 +29,10 @@ func PullCheckout(login *config.Login, repoOwner, repoName string, forceCreateBr
 	if err != nil {
 		return err
 	}
+	if err := workaround.FixPullHeadSha(client, pr); err != nil {
+		return err
+	}
+
 	remoteDeleted := pr.Head.Ref == fmt.Sprintf("refs/pull/%d/head", pr.Index)
 	if remoteDeleted {
 		return fmt.Errorf("Can't checkout: remote head branch was already deleted")
diff --git a/modules/task/pull_clean.go b/modules/task/pull_clean.go
index bcefbbc..0528cb4 100644
--- a/modules/task/pull_clean.go
+++ b/modules/task/pull_clean.go
@@ -9,6 +9,7 @@ import (
 
 	"code.gitea.io/tea/modules/config"
 	local_git "code.gitea.io/tea/modules/git"
+	"code.gitea.io/tea/modules/workaround"
 
 	"code.gitea.io/sdk/gitea"
 	git_config "github.com/go-git/go-git/v5/config"
@@ -33,6 +34,10 @@ func PullClean(login *config.Login, repoOwner, repoName string, index int64, ign
 	if err != nil {
 		return err
 	}
+	if err := workaround.FixPullHeadSha(client, pr); err != nil {
+		return err
+	}
+
 	if pr.State == gitea.StateOpen {
 		return fmt.Errorf("PR is still open, won't delete branches")
 	}
diff --git a/modules/workaround/pull.go b/modules/workaround/pull.go
new file mode 100644
index 0000000..79d5b31
--- /dev/null
+++ b/modules/workaround/pull.go
@@ -0,0 +1,30 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package workaround
+
+import (
+	"fmt"
+
+	"code.gitea.io/sdk/gitea"
+)
+
+// FixPullHeadSha is a workaround for https://github.com/go-gitea/gitea/issues/12675
+// When no head sha is available, this is because the branch got deleted in the base repo.
+// pr.Head.Ref points in this case not to the head repo branch name, but the base repo ref,
+// which stays available to resolve the commit sha.
+func FixPullHeadSha(client *gitea.Client, pr *gitea.PullRequest) error {
+	owner := pr.Base.Repository.Owner.UserName
+	repo := pr.Base.Repository.Name
+	if pr.Head != nil && pr.Head.Sha == "" {
+		refs, _, err := client.GetRepoRefs(owner, repo, pr.Head.Ref)
+		if err != nil {
+			return err
+		} else if len(refs) == 0 {
+			return fmt.Errorf("unable to resolve PR ref '%s'", pr.Head.Ref)
+		}
+		pr.Head.Sha = refs[0].Object.SHA
+	}
+	return nil
+}