diff --git a/internal/frontend/details.go b/internal/frontend/details.go
index 5592665b..f9705160 100644
--- a/internal/frontend/details.go
+++ b/internal/frontend/details.go
@@ -488,7 +488,7 @@ func (s *Server) servePathNotFoundPage(w http.ResponseWriter, r *http.Request, d
 		}
 		modulePaths, err := candidateModulePaths(fullPath)
 		if err != nil {
-			return pathNotFoundError(ctx, pathType, fullPath, requestedVersion)
+			return err
 		}
 		results := s.checkPossibleModulePaths(ctx, db, fullPath, requestedVersion, modulePaths, false)
 		for _, fr := range results {
diff --git a/internal/frontend/fetch.go b/internal/frontend/fetch.go
index 22303593..80b31272 100644
--- a/internal/frontend/fetch.go
+++ b/internal/frontend/fetch.go
@@ -469,6 +469,12 @@ func candidateModulePaths(fullPath string) (_ []string, err error) {
 		modulePaths = append([]string{path}, modulePaths...)
 		path += "/"
 	}
+	if len(modulePaths) == 0 {
+		return nil, &serverError{
+			status: http.StatusBadRequest,
+			err:    fmt.Errorf("invalid path: %q", fullPath),
+		}
+	}
 	return modulePaths, nil
 }
 
diff --git a/internal/frontend/server_test.go b/internal/frontend/server_test.go
index 2784e6eb..3cbe016e 100644
--- a/internal/frontend/server_test.go
+++ b/internal/frontend/server_test.go
@@ -881,6 +881,12 @@ func frontendFetchTestCases() []serverTestCase {
 				in("h3.Fetch-message.js-fetchMessage", text("example.com/unknown"))),
 			requiredExperiments: experiment.NewSet(internal.ExperimentFrontendFetch),
 		},
+		{
+			name:                "bad request, invalid github module path, frontend experiment",
+			urlPath:             "/github.com/foo",
+			wantStatusCode:      http.StatusBadRequest,
+			requiredExperiments: experiment.NewSet(internal.ExperimentFrontendFetch),
+		},
 	}
 }