diff --git a/cmd/server/tests/test_helpers_test.go b/cmd/server/tests/test_helpers_test.go
index 48eb8adf..656f0b7f 100644
--- a/cmd/server/tests/test_helpers_test.go
+++ b/cmd/server/tests/test_helpers_test.go
@@ -572,6 +572,10 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 		// the main binary wires database.NewSettingsRegistry(queries) —
 		// the test harness must too or route registration nil-panics.
 		Settings: database.NewSettingsRegistry(queries),
+		// main.go:246 passes the shared LibraryService here; omitting it
+		// hands the kobo/kepub CFI converter a typed-nil (kobo markup
+		// panic-recovered as 500s in TestProgressWeb_KoboIntegration).
+		LibraryService: libraryService,
 	}
 
 	router.RegisterRoutes(routerConfig)
diff --git a/internal/services/library_service.go b/internal/services/library_service.go
index 9746da6f..9bd4cc65 100644
--- a/internal/services/library_service.go
+++ b/internal/services/library_service.go
@@ -299,6 +299,13 @@ func (s *LibraryService) SyncAllowedExtensions(ctx context.Context) {
 }
 
 func (s *LibraryService) ResolveMediaPath(ctx context.Context, libraryID pgtype.UUID, relativePath string) (string, error) {
+	// Typed-nil interface guard: a nil *LibraryService held in an
+	// interface passes `!= nil` checks at call sites; degrade instead
+	// of panicking (kobo markup 500s were exactly this path).
+	if s == nil || s.db == nil {
+		return "", fmt.Errorf("library service not configured")
+	}
+
 	// Get library folders for this library
 	folders, err := s.db.GetLibraryFolders(ctx, libraryID)
 	if err != nil || len(folders) == 0 {