diff --git a/core/src/main/java/org/elasticsearch/index/IndexingSlowLog.java b/core/src/main/java/org/elasticsearch/index/IndexingSlowLog.java
index b1d8ac188626f..94c3892ef361e 100644
--- a/core/src/main/java/org/elasticsearch/index/IndexingSlowLog.java
+++ b/core/src/main/java/org/elasticsearch/index/IndexingSlowLog.java
@@ -181,9 +181,9 @@ public String toString() {
             sb.append("type[").append(doc.type()).append("], ");
             sb.append("id[").append(doc.id()).append("], ");
             if (doc.routing() == null) {
-                sb.append("routing[] ");
+                sb.append("routing[]");
             } else {
-                sb.append("routing[").append(doc.routing()).append("] ");
+                sb.append("routing[").append(doc.routing()).append("]");
             }
 
             if (maxSourceCharsToLog == 0 || doc.source() == null || doc.source().length() == 0) {
@@ -193,7 +193,7 @@ public String toString() {
                 String source = XContentHelper.convertToJson(doc.source(), reformat, doc.getXContentType());
                 sb.append(", source[").append(Strings.cleanTruncate(source, maxSourceCharsToLog)).append("]");
             } catch (IOException e) {
-                sb.append(", source[_failed_to_convert_]");
+                sb.append(", source[_failed_to_convert_[").append(e.getMessage()).append("]]");
             }
             return sb.toString();
         }
diff --git a/core/src/test/java/org/elasticsearch/index/IndexingSlowLogTests.java b/core/src/test/java/org/elasticsearch/index/IndexingSlowLogTests.java
index a3d14fc518499..45b0d0aa2475c 100644
--- a/core/src/test/java/org/elasticsearch/index/IndexingSlowLogTests.java
+++ b/core/src/test/java/org/elasticsearch/index/IndexingSlowLogTests.java
@@ -22,6 +22,7 @@
 import org.apache.lucene.document.NumericDocValuesField;
 import org.elasticsearch.Version;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
+import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.TimeValue;
@@ -62,6 +63,16 @@ public void testSlowLogParsedDocumentPrinterSourceToLog() throws IOException {
         p = new SlowLogParsedDocumentPrinter(index, pd, 10, true, 3);
         assertThat(p.toString(), containsString("source[{\"f]"));
         assertThat(p.toString(), startsWith("[foo/123] took"));
+
+        // Throwing a error if source cannot be converted
+        source = new BytesArray("invalid");
+        pd = new ParsedDocument(new NumericDocValuesField("version", 1), SeqNoFieldMapper.SequenceIDFields.emptySeqID(), "id",
+            "test", null, null, source, XContentType.JSON, null);
+        p = new SlowLogParsedDocumentPrinter(index, pd, 10, true, 3);
+
+        assertThat(p.toString(), containsString("_failed_to_convert_[Unrecognized token 'invalid':"
+            + " was expecting ('true', 'false' or 'null')\n"
+            + " at [Source: org.elasticsearch.common.bytes.BytesReference$MarkSupportingStreamInputWrapper"));
     }
 
     public void testReformatSetting() {