diff --git a/cmd/tusd/cli/flags.go b/cmd/tusd/cli/flags.go
index 5e80efb..221f231 100644
--- a/cmd/tusd/cli/flags.go
+++ b/cmd/tusd/cli/flags.go
@@ -3,6 +3,7 @@ package cli
 import (
 	"flag"
 	"path/filepath"
+	"strings"
 )
 
 var Flags struct {
@@ -45,7 +46,7 @@ func ParseFlags() {
 	flag.StringVar(&Flags.S3ObjectPrefix, "s3-object-prefix", "", "Prefix for S3 object names")
 	flag.StringVar(&Flags.S3Endpoint, "s3-endpoint", "", "Endpoint to use S3 compatible implementations like minio (requires s3-bucket to be pass)")
 	flag.StringVar(&Flags.GCSBucket, "gcs-bucket", "", "Use Google Cloud Storage with this bucket as storage backend (requires the GCS_SERVICE_ACCOUNT_FILE environment variable to be set)")
-	flag.StringVar(&Flags.GCSObjectPrefix, "gcs-object-prefix", "", "Prefix for GCS object names")
+	flag.StringVar(&Flags.GCSObjectPrefix, "gcs-object-prefix", "", "Prefix for GCS object names (can't contain underscore character)")
 	flag.StringVar(&Flags.FileHooksDir, "hooks-dir", "", "Directory to search for available hooks scripts")
 	flag.StringVar(&Flags.HttpHooksEndpoint, "hooks-http", "", "An HTTP endpoint to which hook events will be sent to")
 	flag.IntVar(&Flags.HttpHooksRetry, "hooks-http-retry", 3, "Number of times to retry on a 500 or network timeout")
@@ -76,4 +77,9 @@ func ParseFlags() {
 			"neither flag was provided. Please consult `tusd -help` for " +
 			"more information on these options.")
 	}
+
+	if Flags.GCSObjectPrefix != "" && strings.Contains(Flags.GCSObjectPrefix, "_") {
+		stderr.Fatalf("gcs-object-prefix value (%s) can't contain underscore. "+
+			"Please remove underscore from the value", Flags.GCSObjectPrefix)
+	}
 }