diff --git a/internal/lsp/cache/check.go b/internal/lsp/cache/check.go
index 67b9256682..24933a5b51 100644
--- a/internal/lsp/cache/check.go
+++ b/internal/lsp/cache/check.go
@@ -15,11 +15,10 @@ import (
 
 	"golang.org/x/tools/go/analysis"
 	"golang.org/x/tools/go/packages"
-	"golang.org/x/tools/internal/lsp/source"
 	"golang.org/x/tools/internal/span"
 )
 
-func (v *view) parse(ctx context.Context, file source.File) ([]packages.Error, error) {
+func (v *view) parse(ctx context.Context, f *goFile) ([]packages.Error, error) {
 	v.mcache.mu.Lock()
 	defer v.mcache.mu.Unlock()
 
@@ -28,11 +27,6 @@ func (v *view) parse(ctx context.Context, file source.File) ([]packages.Error, e
 		return nil, err
 	}
 
-	f, ok := file.(*goFile)
-	if !ok {
-		return nil, fmt.Errorf("not a go file: %v", file.URI())
-	}
-
 	// If the package for the file has not been invalidated by the application
 	// of the pending changes, there is no need to continue.
 	if f.isPopulated() {
diff --git a/internal/lsp/cache/view.go b/internal/lsp/cache/view.go
index f2573605a4..1cf37d0f09 100644
--- a/internal/lsp/cache/view.go
+++ b/internal/lsp/cache/view.go
@@ -6,10 +6,12 @@ package cache
 
 import (
 	"context"
+	"fmt"
 	"go/ast"
 	"go/parser"
 	"go/types"
 	"os"
+	"path/filepath"
 	"sync"
 
 	"golang.org/x/tools/go/packages"
@@ -327,12 +329,21 @@ func (v *view) getFile(uri span.URI) (viewFile, error) {
 	if err != nil {
 		return nil, err
 	}
-	f := &goFile{
-		fileBase: fileBase{
-			view:  v,
-			fname: filename,
-		},
+	var f *goFile
+	switch ext := filepath.Ext(filename); ext {
+	case ".go":
+		f = &goFile{
+			fileBase: fileBase{
+				view:  v,
+				fname: filename,
+			},
+		}
+	case ".mod":
+		return nil, fmt.Errorf("mod files are not yet supported")
+	default:
+		return nil, fmt.Errorf("unsupported file extension: %s", ext)
 	}
+
 	v.mapFile(uri, f)
 	return f, nil
 }
diff --git a/internal/lsp/source/diagnostics.go b/internal/lsp/source/diagnostics.go
index 5cbea3ccbd..2b4757ca5c 100644
--- a/internal/lsp/source/diagnostics.go
+++ b/internal/lsp/source/diagnostics.go
@@ -56,9 +56,10 @@ func Diagnostics(ctx context.Context, v View, uri span.URI) (map[span.URI][]Diag
 	if err != nil {
 		return singleDiagnostic(uri, "no file found for %s", uri), nil
 	}
+	// For non-Go files, don't return any diagnostics.
 	gof, ok := f.(GoFile)
 	if !ok {
-		return singleDiagnostic(uri, "%s is not a go file", uri), nil
+		return nil, nil
 	}
 	pkg := gof.GetPackage(ctx)
 	if pkg == nil {