diff --git a/gnu/packages/graphviz.scm b/gnu/packages/graphviz.scm
index 34aa1dc470..dd5d2de87b 100644
--- a/gnu/packages/graphviz.scm
+++ b/gnu/packages/graphviz.scm
@@ -194,6 +194,7 @@ (define-public python-graphviz
      (list unzip
            ;; For tests.
            graphviz
+           python-mock
            python-pytest
            python-pytest-cov
            python-pytest-mock))