diff --git a/src/Cpdf.php b/src/Cpdf.php
index daf710d..eaa28dd 100644
--- a/src/Cpdf.php
+++ b/src/Cpdf.php
@@ -2232,7 +2232,7 @@ public function selectFont($fontName, $encoding = '', $set = 1, $subsetFont = fa
                 // also set the differences here, note that this means that these will take effect only the
                 // first time that a font is selected, else they are ignored
                 if (isset($options['differences'])) {
-                    $font['differences'] = $options['differences'];
+                    $this->fonts[$fontName]['differences'] = $options['differences'];
                 }
             }
         }