diff --git a/models/user/search.go b/models/user/search.go
index 558daf2e6a..cb90ca850e 100644
--- a/models/user/search.go
+++ b/models/user/search.go
@@ -24,8 +24,8 @@ type SearchUserOptions struct {
 	Keyword       string
 	Type          UserType
 	UID           int64
-	LoginName     string // this option should be used only for admin user
-	SourceID      int64  // this option should be used only for admin user
+	LoginName     string                 // this option should be used only for admin user
+	SourceID      optional.Option[int64] // this option should be used only for admin user
 	OrderBy       db.SearchOrderBy
 	Visible       []structs.VisibleType
 	Actor         *User // The user doing the search
@@ -98,8 +98,8 @@ func (opts *SearchUserOptions) toSearchQueryBase(ctx context.Context) *xorm.Sess
 		cond = cond.And(builder.Eq{"id": opts.UID})
 	}
 
-	if opts.SourceID > 0 {
-		cond = cond.And(builder.Eq{"login_source": opts.SourceID})
+	if opts.SourceID.Has() {
+		cond = cond.And(builder.Eq{"login_source": opts.SourceID.Value()})
 	}
 	if opts.LoginName != "" {
 		cond = cond.And(builder.Eq{"login_name": opts.LoginName})
diff --git a/routers/api/v1/admin/user.go b/routers/api/v1/admin/user.go
index b17200381a..184024a246 100644
--- a/routers/api/v1/admin/user.go
+++ b/routers/api/v1/admin/user.go
@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strconv"
 
 	"code.gitea.io/gitea/models"
 	asymkey_model "code.gitea.io/gitea/models/asymkey"
@@ -430,12 +431,19 @@ func SearchUsers(ctx *context.APIContext) {
 	//     "$ref": "#/responses/forbidden"
 
 	listOptions := utils.GetListOptions(ctx)
+	intSource, err := strconv.ParseInt(ctx.FormString("source_id"), 10, 64)
+	var sourceID optional.Option[int64]
+	if ctx.FormString("source_id") == "" || err != nil {
+		sourceID = optional.None[int64]()
+	} else {
+		sourceID = optional.Some(intSource)
+	}
 
 	users, maxResults, err := user_model.SearchUsers(ctx, &user_model.SearchUserOptions{
 		Actor:       ctx.Doer,
 		Type:        user_model.UserTypeIndividual,
 		LoginName:   ctx.FormTrim("login_name"),
-		SourceID:    ctx.FormInt64("source_id"),
+		SourceID:    sourceID,
 		OrderBy:     db.SearchOrderByAlphabetically,
 		ListOptions: listOptions,
 	})
diff --git a/tests/integration/admin_user_test.go b/tests/integration/admin_user_test.go
index 8cdaac3c72..e61a46ab07 100644
--- a/tests/integration/admin_user_test.go
+++ b/tests/integration/admin_user_test.go
@@ -4,14 +4,17 @@
 package integration
 
 import (
+	"context"
 	"fmt"
 	"net/http"
 	"strconv"
 	"testing"
 
+	auth_model "code.gitea.io/gitea/models/auth"
 	issues_model "code.gitea.io/gitea/models/issues"
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
+	api "code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/tests"
 
 	"github.com/stretchr/testify/assert"
@@ -89,3 +92,43 @@ func TestAdminDeleteUser(t *testing.T) {
 	assertUserDeleted(t, userID, true)
 	unittest.CheckConsistencyFor(t, &user_model.User{})
 }
+
+func TestSourceId(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	testUser23 := &user_model.User{
+		Name:        "ausersourceid23",
+		LoginName:   "ausersourceid23",
+		Email:       "ausersourceid23@example.com",
+		Passwd:      "ausersourceid23password",
+		Type:        user_model.UserTypeIndividual,
+		LoginType:   auth_model.Plain,
+		LoginSource: 23,
+	}
+	defer createUser(context.Background(), t, testUser23)()
+
+	session := loginUser(t, "user1")
+	token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeReadAdmin)
+
+	// Our new user start with 'a' so it should be the first one
+	req := NewRequest(t, "GET", "/api/v1/admin/users?limit=1").AddTokenAuth(token)
+	resp := session.MakeRequest(t, req, http.StatusOK)
+	var users []api.User
+	DecodeJSON(t, resp, &users)
+	assert.Len(t, users, 1)
+	assert.Equal(t, "ausersourceid23", users[0].UserName)
+
+	// Now our new user should not be in the list, because we filter by source_id 0
+	req = NewRequest(t, "GET", "/api/v1/admin/users?limit=1&source_id=0").AddTokenAuth(token)
+	resp = session.MakeRequest(t, req, http.StatusOK)
+	DecodeJSON(t, resp, &users)
+	assert.Len(t, users, 1)
+	assert.Equal(t, "the_34-user.with.all.allowedChars", users[0].UserName)
+
+	// Now our new user should be in the list, because we filter by source_id 23
+	req = NewRequest(t, "GET", "/api/v1/admin/users?limit=1&source_id=23").AddTokenAuth(token)
+	resp = session.MakeRequest(t, req, http.StatusOK)
+	DecodeJSON(t, resp, &users)
+	assert.Len(t, users, 1)
+	assert.Equal(t, "ausersourceid23", users[0].UserName)
+}