diff --git a/fs/fs_context.c b/fs/fs_context.c
index e492a83fa100..412712eb59ee 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -598,7 +598,8 @@ static int legacy_parse_param(struct fs_context *fc, struct fs_parameter *param)
 			return -ENOMEM;
 	}
 
-	ctx->legacy_data[size++] = ',';
+	if (size)
+		ctx->legacy_data[size++] = ',';
 	len = strlen(param->key);
 	memcpy(ctx->legacy_data + size, param->key, len);
 	size += len;