diff --git a/internal/lsp/cache/session.go b/internal/lsp/cache/session.go
index 920eac24..1b4881bb 100644
--- a/internal/lsp/cache/session.go
+++ b/internal/lsp/cache/session.go
@@ -78,6 +78,10 @@ func (s *session) NewView(name string, folder span.URI) source.View {
 		},
 		ignoredURIs: make(map[span.URI]struct{}),
 	}
+	// Preemptively build the builtin package,
+	// so we immediately add builtin.go to the list of ignored files.
+	v.buildBuiltinPkg()
+
 	s.views = append(s.views, v)
 	// we always need to drop the view map
 	s.viewMap = make(map[span.URI]source.View)
diff --git a/internal/lsp/cache/view.go b/internal/lsp/cache/view.go
index b2bb3e61..2aef2237 100644
--- a/internal/lsp/cache/view.go
+++ b/internal/lsp/cache/view.go
@@ -177,15 +177,13 @@ func (v *view) BackgroundContext() context.Context {
 }
 
 func (v *view) BuiltinPackage() *ast.Package {
-	if v.builtinPkg == nil {
-		v.buildBuiltinPkg()
-	}
 	return v.builtinPkg
 }
 
+// buildBuiltinPkg builds the view's builtin package.
+// It assumes that the view is not active yet,
+// i.e. it has not been added to the session's list of views.
 func (v *view) buildBuiltinPkg() {
-	v.mu.Lock()
-	defer v.mu.Unlock()
 	cfg := *v.buildConfig()
 	pkgs, _ := packages.Load(&cfg, "builtin")
 	if len(pkgs) != 1 {
diff --git a/internal/lsp/source/diagnostics.go b/internal/lsp/source/diagnostics.go
index bd0cd2d7..01056019 100644
--- a/internal/lsp/source/diagnostics.go
+++ b/internal/lsp/source/diagnostics.go
@@ -59,16 +59,12 @@ func Diagnostics(ctx context.Context, v View, f GoFile) (map[span.URI][]Diagnost
 	// Prepare the reports we will send for the files in this package.
 	reports := make(map[span.URI][]Diagnostic)
 	for _, filename := range pkg.GetFilenames() {
-		uri := span.FileURI(filename)
-		if v.Ignore(uri) {
-			continue
-		}
-		reports[uri] = []Diagnostic{}
+		addReport(v, reports, span.FileURI(filename), nil)
 	}
 
 	// Prepare any additional reports for the errors in this package.
 	for _, pkgErr := range pkg.GetErrors() {
-		reports[packageErrorSpan(pkgErr).URI()] = []Diagnostic{}
+		addReport(v, reports, packageErrorSpan(pkgErr).URI(), nil)
 	}
 
 	// Run diagnostics for the package that this URI belongs to.
@@ -85,7 +81,7 @@ func Diagnostics(ctx context.Context, v View, f GoFile) (map[span.URI][]Diagnost
 			continue
 		}
 		for _, filename := range pkg.GetFilenames() {
-			reports[span.FileURI(filename)] = []Diagnostic{}
+			addReport(v, reports, span.FileURI(filename), nil)
 		}
 		diagnostics(ctx, v, pkg, reports)
 	}
@@ -143,7 +139,7 @@ func analyses(ctx context.Context, v View, pkg Package, reports map[span.URI][]D
 		if diag.Category != "" {
 			category += "." + category
 		}
-		reports[s.URI()] = append(reports[s.URI()], Diagnostic{
+		addReport(v, reports, s.URI(), &Diagnostic{
 			Source:   category,
 			Span:     s,
 			Message:  diag.Message,
@@ -156,6 +152,17 @@ func analyses(ctx context.Context, v View, pkg Package, reports map[span.URI][]D
 	return nil
 }
 
+func addReport(v View, reports map[span.URI][]Diagnostic, uri span.URI, diagnostic *Diagnostic) {
+	if v.Ignore(uri) {
+		return
+	}
+	if diagnostic == nil {
+		reports[uri] = []Diagnostic{}
+	} else {
+		reports[uri] = append(reports[uri], *diagnostic)
+	}
+}
+
 // parseDiagnosticMessage attempts to parse a standard error message by stripping off the trailing error message.
 // Works only on errors where the message is prefixed by ": ".
 // e.g.: