diff --git a/src/PVE/Storage/BTRFSPlugin.pm b/src/PVE/Storage/BTRFSPlugin.pm
index 555ed9d..def51ef 100644
--- a/src/PVE/Storage/BTRFSPlugin.pm
+++ b/src/PVE/Storage/BTRFSPlugin.pm
@@ -678,6 +678,7 @@ sub list_images {
 		warn "image '$fn' is not in expected format '$format', querying as raw\n";
 		($size, undef, $used, $parent, $ctime) =
 		    PVE::Storage::Plugin::file_size_info($fn, undef, 'raw');
+		$format = 'invalid';
 	    }
 	}
 	next if !defined($size);
diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
index 917bd61..ae3c9dc 100644
--- a/src/PVE/Storage/Plugin.pm
+++ b/src/PVE/Storage/Plugin.pm
@@ -1300,6 +1300,7 @@ sub list_images {
 	    die $err if $err !~ m/Image is not in \S+ format$/;
 	    warn "image '$fn' is not in expected format '$format', querying as raw\n";
 	    ($size, undef, $used, $parent, $ctime) = file_size_info($fn, undef, 'raw');
+	    $format = 'invalid';
 	}
 	next if !defined($size);