diff --git a/pkg/constants/constants.go b/pkg/constants/constants.go
index abde570..3bf611c 100644
--- a/pkg/constants/constants.go
+++ b/pkg/constants/constants.go
@@ -120,6 +120,9 @@ const (
 	PassiveBootSuffix    = " (fallback)"
 	RecoveryBootSuffix   = " recovery"
 	StateResetBootSuffix = " state reset (auto)"
+
+	// Error
+	UpgradeNoSourceError = "Could not find a proper source for the upgrade.\nThis can be configured in the cloud config files under the 'upgrade.system.uri' key or via cmdline using the '--source' flag."
 )
 
 func UkiDefaultMenuEntries() []string {
diff --git a/pkg/types/v1/config.go b/pkg/types/v1/config.go
index e1c6eb5..d16bf89 100644
--- a/pkg/types/v1/config.go
+++ b/pkg/types/v1/config.go
@@ -187,14 +187,14 @@ type UpgradeSpec struct {
 func (u *UpgradeSpec) Sanitize() error {
 	if u.RecoveryUpgrade {
 		if u.Recovery.Source.IsEmpty() {
-			return fmt.Errorf("undefined upgrade source")
+			return fmt.Errorf(constants.UpgradeNoSourceError)
 		}
 		if u.Partitions.Recovery == nil || u.Partitions.Recovery.MountPoint == "" {
 			return fmt.Errorf("undefined recovery partition")
 		}
 	} else {
 		if u.Active.Source.IsEmpty() {
-			return fmt.Errorf("undefined upgrade source")
+			return fmt.Errorf(constants.UpgradeNoSourceError)
 		}
 		if u.Partitions.State == nil || u.Partitions.State.MountPoint == "" {
 			return fmt.Errorf("undefined state partition")
diff --git a/pkg/types/v1/config_test.go b/pkg/types/v1/config_test.go
index 80916f0..877e7c1 100644
--- a/pkg/types/v1/config_test.go
+++ b/pkg/types/v1/config_test.go
@@ -425,7 +425,7 @@ var _ = Describe("Types", Label("types", "config"), func() {
 				It("fails with empty source", func() {
 					err := spec.Sanitize()
 					Expect(err).To(HaveOccurred())
-					Expect(err.Error()).To(ContainSubstring("undefined upgrade source"))
+					Expect(err.Error()).To(ContainSubstring(constants.UpgradeNoSourceError))
 				})
 				It("fails with missing state partition", func() {
 					spec.Active.Source = v1.NewFileSrc("/tmp")
@@ -449,7 +449,7 @@ var _ = Describe("Types", Label("types", "config"), func() {
 				It("fails with empty source", func() {
 					err := spec.Sanitize()
 					Expect(err).To(HaveOccurred())
-					Expect(err.Error()).To(ContainSubstring("undefined upgrade source"))
+					Expect(err.Error()).To(ContainSubstring(constants.UpgradeNoSourceError))
 				})
 				It("fails with missing recovery partition", func() {
 					spec.Recovery.Source = v1.NewFileSrc("/tmp")