diff --git a/models/git/branch.go b/models/git/branch.go
index d1caa35947..9ac6c45578 100644
--- a/models/git/branch.go
+++ b/models/git/branch.go
@@ -173,6 +173,18 @@ func GetBranch(ctx context.Context, repoID int64, branchName string) (*Branch, e
 	return &branch, nil
 }
 
+// IsBranchExist returns true if the branch exists in the repository.
+func IsBranchExist(ctx context.Context, repoID int64, branchName string) (bool, error) {
+	var branch Branch
+	has, err := db.GetEngine(ctx).Where("repo_id=?", repoID).And("name=?", branchName).Get(&branch)
+	if err != nil {
+		return false, err
+	} else if !has {
+		return false, nil
+	}
+	return !branch.IsDeleted, nil
+}
+
 func GetBranches(ctx context.Context, repoID int64, branchNames []string, includeDeleted bool) ([]*Branch, error) {
 	branches := make([]*Branch, 0, len(branchNames))
 
diff --git a/options/locale/locale_en-US.ini b/options/locale/locale_en-US.ini
index 9d71ccb6d5..e28840277a 100644
--- a/options/locale/locale_en-US.ini
+++ b/options/locale/locale_en-US.ini
@@ -2699,6 +2699,7 @@ branch.restore_success = Branch "%s" has been restored.
 branch.restore_failed = Failed to restore branch "%s".
 branch.protected_deletion_failed = Branch "%s" is protected. It cannot be deleted.
 branch.default_deletion_failed = Branch "%s" is the default branch. It cannot be deleted.
+branch.default_branch_not_exist = Default branch "%s" does not exist.
 branch.restore = Restore Branch "%s"
 branch.download = Download Branch "%s"
 branch.rename = Rename Branch "%s"
diff --git a/routers/web/repo/actions/actions.go b/routers/web/repo/actions/actions.go
index 1de1835936..46b7461883 100644
--- a/routers/web/repo/actions/actions.go
+++ b/routers/web/repo/actions/actions.go
@@ -6,6 +6,7 @@ package actions
 import (
 	"bytes"
 	stdCtx "context"
+	"errors"
 	"fmt"
 	"net/http"
 	"slices"
@@ -77,7 +78,11 @@ func List(ctx *context.Context) {
 		return
 	} else if !empty {
 		commit, err := ctx.Repo.GitRepo.GetBranchCommit(ctx.Repo.Repository.DefaultBranch)
-		if err != nil {
+		if errors.Is(err, util.ErrNotExist) {
+			ctx.Data["NotFoundPrompt"] = ctx.Tr("repo.branch.default_branch_not_exist", ctx.Repo.Repository.DefaultBranch)
+			ctx.NotFound("GetBranchCommit", err)
+			return
+		} else if err != nil {
 			ctx.ServerError("GetBranchCommit", err)
 			return
 		}
diff --git a/routers/web/repo/activity.go b/routers/web/repo/activity.go
index 65dd9e392f..65fd379406 100644
--- a/routers/web/repo/activity.go
+++ b/routers/web/repo/activity.go
@@ -8,6 +8,7 @@ import (
 	"time"
 
 	activities_model "code.gitea.io/gitea/models/activities"
+	"code.gitea.io/gitea/models/git"
 	"code.gitea.io/gitea/models/unit"
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/services/context"
@@ -52,12 +53,26 @@ func Activity(ctx *context.Context) {
 	ctx.Data["DateUntil"] = timeUntil
 	ctx.Data["PeriodText"] = ctx.Tr("repo.activity.period." + ctx.Data["Period"].(string))
 
+	canReadCode := ctx.Repo.CanRead(unit.TypeCode)
+	if canReadCode {
+		// GetActivityStats needs to read the default branch to get some information
+		branchExist, _ := git.IsBranchExist(ctx, ctx.Repo.Repository.ID, ctx.Repo.Repository.DefaultBranch)
+		if !branchExist {
+			ctx.Data["NotFoundPrompt"] = ctx.Tr("repo.branch.default_branch_not_exist", ctx.Repo.Repository.DefaultBranch)
+			ctx.NotFound("", nil)
+			return
+		}
+	}
+
 	var err error
-	if ctx.Data["Activity"], err = activities_model.GetActivityStats(ctx, ctx.Repo.Repository, timeFrom,
+	// TODO: refactor these arguments to a struct
+	ctx.Data["Activity"], err = activities_model.GetActivityStats(ctx, ctx.Repo.Repository, timeFrom,
 		ctx.Repo.CanRead(unit.TypeReleases),
 		ctx.Repo.CanRead(unit.TypeIssues),
 		ctx.Repo.CanRead(unit.TypePullRequests),
-		ctx.Repo.CanRead(unit.TypeCode)); err != nil {
+		canReadCode,
+	)
+	if err != nil {
 		ctx.ServerError("GetActivityStats", err)
 		return
 	}
diff --git a/services/context/context_response.go b/services/context/context_response.go
index c43a649b49..14a31dad84 100644
--- a/services/context/context_response.go
+++ b/services/context/context_response.go
@@ -151,7 +151,7 @@ func (ctx *Context) notFoundInternal(logMsg string, logErr error) {
 
 	ctx.Data["IsRepo"] = ctx.Repo.Repository != nil
 	ctx.Data["Title"] = "Page Not Found"
-	ctx.HTML(http.StatusNotFound, base.TplName("status/404"))
+	ctx.HTML(http.StatusNotFound, "status/404")
 }
 
 // ServerError displays a 500 (Internal Server Error) page and prints the given error, if any.
diff --git a/tests/integration/repo_activity_test.go b/tests/integration/repo_activity_test.go
index b04560379d..e520a0ed56 100644
--- a/tests/integration/repo_activity_test.go
+++ b/tests/integration/repo_activity_test.go
@@ -4,12 +4,15 @@
 package integration
 
 import (
+	"context"
 	"net/http"
 	"net/url"
 	"strings"
 	"testing"
 
+	"code.gitea.io/gitea/models/db"
 	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/models/unittest"
 	"code.gitea.io/gitea/modules/test"
 
 	"github.com/stretchr/testify/assert"
@@ -61,5 +64,14 @@ func TestRepoActivity(t *testing.T) {
 		// Should be 3 new issues
 		list = htmlDoc.doc.Find("#new-issues").Next().Find("p.desc")
 		assert.Len(t, list.Nodes, 3)
+
+		// Non-existing default branch
+		repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{Name: "repo1"})
+		repo1.DefaultBranch = "no-such-branch"
+		_, _ = db.GetEngine(context.Background()).Cols("default_branch").Update(repo1)
+		req = NewRequest(t, "GET", "/user2/repo1/activity")
+		req.Header.Add("Accept", "text/html")
+		resp = session.MakeRequest(t, req, http.StatusNotFound)
+		assert.Contains(t, resp.Body.String(), `Default branch "no-such-branch" does not exist.`)
 	})
 }