diff --git a/src/main/java/org/embulk/output/sftp/SftpFileOutput.java b/src/main/java/org/embulk/output/sftp/SftpFileOutput.java
index 3379c85..e3c2505 100644
--- a/src/main/java/org/embulk/output/sftp/SftpFileOutput.java
+++ b/src/main/java/org/embulk/output/sftp/SftpFileOutput.java
@@ -7,6 +7,7 @@
 import org.apache.commons.vfs2.impl.StandardFileSystemManager;
 import org.apache.commons.vfs2.provider.sftp.IdentityInfo;
 import org.apache.commons.vfs2.provider.sftp.SftpFileSystemConfigBuilder;
+import org.embulk.config.ConfigException;
 import org.embulk.config.TaskReport;
 import org.embulk.spi.Buffer;
 import org.embulk.spi.Exec;
@@ -56,7 +57,7 @@ private StandardFileSystemManager initializeStandardFileSystemManager()
         }
         catch (FileSystemException e) {
             logger.error(e.getMessage());
-            throw new RuntimeException(e);
+            throw new ConfigException(e);
         }
 
         return manager;
@@ -87,7 +88,7 @@ private FileSystemOptions initializeFsOptions(PluginTask task)
         }
         catch (FileSystemException e) {
             logger.error(e.getMessage());
-            throw new RuntimeException(e);
+            throw new ConfigException(e);
         }
 
         return fsOptions;
@@ -137,7 +138,9 @@ public void add(Buffer buffer)
             logger.error(e.getMessage());
             Throwables.propagate(e);
         }
-        buffer.release();
+        finally {
+            buffer.release();
+        }
     }
 
     @Override
@@ -161,7 +164,7 @@ public void abort()
     @Override
     public TaskReport commit()
     {
-        return null;
+        return Exec.newTaskReport();
     }
 
 
@@ -194,7 +197,7 @@ private URI getSftpFileUri(String remoteFilePath)
         }
         catch (URISyntaxException e) {
             logger.error(e.getMessage());
-            throw new RuntimeException(e);
+            throw new ConfigException(e);
         }
     }