diff --git a/zxlive/mainwindow.py b/zxlive/mainwindow.py
index dceabe45..4180ec6b 100644
--- a/zxlive/mainwindow.py
+++ b/zxlive/mainwindow.py
@@ -392,6 +392,7 @@ def handle_export_tikz_proof_action(self) -> bool:
             return False
         with open(path, "w") as f:
             f.write(proof_to_tikz(self.active_panel.proof_model))
+        return True
 
     def cut_graph(self) -> None:
         assert self.active_panel is not None
diff --git a/zxlive/tikz.py b/zxlive/tikz.py
index 2dc12fc0..237c1b4e 100644
--- a/zxlive/tikz.py
+++ b/zxlive/tikz.py
@@ -11,6 +11,8 @@ def proof_to_tikz(proof: ProofModel) -> str:
     max_width = settings.value("tikz/layout/max-width")
     draw_scalar = False
 
+    assert isinstance(vspace, float) and isinstance(hspace, float) and isinstance(max_width, float)
+
     xoffset = -max_width
     yoffset = -10
     idoffset = 0