diff --git a/commands/instances.go b/commands/instances.go
index 368f54813ce..d5534494bae 100644
--- a/commands/instances.go
+++ b/commands/instances.go
@@ -534,9 +534,9 @@ func (s *arduinoCoreServerImpl) UpdateIndex(req *rpc.UpdateIndexRequest, stream
 		return &cmderrors.InvalidInstanceError{}
 	}
 
-	report := func(indexURL *url.URL, status rpc.IndexUpdateReport_Status) *rpc.IndexUpdateReport {
+	report := func(indexURL string, status rpc.IndexUpdateReport_Status) *rpc.IndexUpdateReport {
 		return &rpc.IndexUpdateReport{
-			IndexUrl: indexURL.String(),
+			IndexUrl: indexURL,
 			Status:   status,
 		}
 	}
@@ -564,7 +564,7 @@ func (s *arduinoCoreServerImpl) UpdateIndex(req *rpc.UpdateIndexRequest, stream
 			downloadCB.Start(u, i18n.Tr("Downloading index: %s", u))
 			downloadCB.End(false, msg)
 			failed = true
-			result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(URL, rpc.IndexUpdateReport_STATUS_FAILED))
+			result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(u, rpc.IndexUpdateReport_STATUS_FAILED))
 			continue
 		}
 
@@ -582,9 +582,9 @@ func (s *arduinoCoreServerImpl) UpdateIndex(req *rpc.UpdateIndexRequest, stream
 				downloadCB.Start(u, i18n.Tr("Downloading index: %s", filepath.Base(URL.Path)))
 				downloadCB.End(false, msg)
 				failed = true
-				result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(URL, rpc.IndexUpdateReport_STATUS_FAILED))
+				result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(u, rpc.IndexUpdateReport_STATUS_FAILED))
 			} else {
-				result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(URL, rpc.IndexUpdateReport_STATUS_SKIPPED))
+				result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(u, rpc.IndexUpdateReport_STATUS_SKIPPED))
 			}
 			continue
 		}
@@ -596,14 +596,14 @@ func (s *arduinoCoreServerImpl) UpdateIndex(req *rpc.UpdateIndexRequest, stream
 			downloadCB.Start(u, i18n.Tr("Downloading index: %s", filepath.Base(URL.Path)))
 			downloadCB.End(false, i18n.Tr("Invalid index URL: %s", err))
 			failed = true
-			result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(URL, rpc.IndexUpdateReport_STATUS_FAILED))
+			result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(u, rpc.IndexUpdateReport_STATUS_FAILED))
 			continue
 		}
 		indexFile := indexpath.Join(indexFileName)
 		if info, err := indexFile.Stat(); err == nil {
 			ageSecs := int64(time.Since(info.ModTime()).Seconds())
 			if ageSecs < req.GetUpdateIfOlderThanSecs() {
-				result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(URL, rpc.IndexUpdateReport_STATUS_ALREADY_UP_TO_DATE))
+				result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(u, rpc.IndexUpdateReport_STATUS_ALREADY_UP_TO_DATE))
 				continue
 			}
 		}
@@ -622,9 +622,9 @@ func (s *arduinoCoreServerImpl) UpdateIndex(req *rpc.UpdateIndexRequest, stream
 		}
 		if err := indexResource.Download(stream.Context(), indexpath, downloadCB, config); err != nil {
 			failed = true
-			result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(URL, rpc.IndexUpdateReport_STATUS_FAILED))
+			result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(u, rpc.IndexUpdateReport_STATUS_FAILED))
 		} else {
-			result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(URL, rpc.IndexUpdateReport_STATUS_UPDATED))
+			result.UpdatedIndexes = append(result.GetUpdatedIndexes(), report(u, rpc.IndexUpdateReport_STATUS_UPDATED))
 		}
 	}
 	syncSend.Send(&rpc.UpdateIndexResponse{
diff --git a/internal/integrationtest/config/config_test.go b/internal/integrationtest/config/config_test.go
index bd19d0545c2..d7b4104a095 100644
--- a/internal/integrationtest/config/config_test.go
+++ b/internal/integrationtest/config/config_test.go
@@ -944,3 +944,18 @@ func TestI18N(t *testing.T) {
 	require.NoError(t, err)
 	require.Contains(t, string(out), "Available Commands")
 }
+
+func TestCoreUpdateWithInvalidIndexURL(t *testing.T) {
+	// https://github.com/arduino/arduino-cli/issues/2786
+	env, cli := integrationtest.CreateArduinoCLIWithEnvironment(t)
+	t.Cleanup(env.CleanUp)
+
+	_, _, err := cli.Run("config", "init")
+	require.NoError(t, err)
+	_, _, err = cli.Run("config", "set", "board_manager.additional_urls", "foo=https://espressif.github.io/arduino-esp32/package_esp32_index.json")
+	require.NoError(t, err)
+	_, stdErr, err := cli.Run("core", "update-index")
+	require.Error(t, err)
+	require.Contains(t, string(stdErr), `Error initializing instance: Some indexes could not be updated.`)
+	require.Contains(t, string(stdErr), `Invalid additional URL: parse "foo=https://espressif.github.io/arduino-esp32/package_esp32_index.json"`)
+}

<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Transitional//EN' 'http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd'>
<html xmlns='http://www.w3.org/1999/xhtml'>
<head>
<title>pFad - Phonifier reborn</title>
<meta http-equiv='Content-Type' content='text/html; charset=utf-8' />
</head>
<body>
<h1>Pfad - The Proxy pFad of &#169; 2024 Garber Painting. All rights reserved.</h1>


<!-- Disclaimer -->
<p>Note: This service is not intended for secure transactions such as banking, social media, email, or purchasing. Use at your own risk. We assume no liability whatsoever for broken pages.</p>
<br>
<p>Alternative Proxies:</p><p><a href="http://rainy.clevelandohioweatherforecast.com/php-proxy/index.php?q=https://patch-diff.githubusercontent.com/raw/arduino/arduino-cli/pull/2817.diff" target="_blank">Alternative Proxy</a></p><p><a href="http://rainy.clevelandohioweatherforecast.com/pFad/index.php?u=https://patch-diff.githubusercontent.com/raw/arduino/arduino-cli/pull/2817.diff" target="_blank">pFad Proxy</a></p><p><a href="http://rainy.clevelandohioweatherforecast.com/pFad/v3index.php?u=https://patch-diff.githubusercontent.com/raw/arduino/arduino-cli/pull/2817.diff" target="_blank">pFad v3 Proxy</a></p><p><a href="http://rainy.clevelandohioweatherforecast.com/pFad/v4index.php?u=https://patch-diff.githubusercontent.com/raw/arduino/arduino-cli/pull/2817.diff" target="_blank">pFad v4 Proxy</a></p></body>
</html>