diff --git a/includes/Image.php b/includes/Image.php
index 0f2251c83f9..8dba559c050 100644
--- a/includes/Image.php
+++ b/includes/Image.php
@@ -1169,9 +1169,11 @@ class Image
 				unlink( $thumbPath );
 			} else {
 				// All good
-				wfDebugLog( 'thumbnail',
-					sprintf( 'thumbnail maybe failed on %s, non-empty output kept: "%s" from "%s"',
-						wfHostname(), trim($err), $cmd ) );
+				if( $err !== true ) {
+					wfDebugLog( 'thumbnail',
+						sprintf( 'thumbnail maybe failed on %s, non-empty output kept: "%s" from "%s"',
+							wfHostname(), trim($err), $cmd ) );
+				}
 				$err = true;
 			}
 		}