diff --git a/src/subs.rs b/src/subs.rs
index 52409c191a..aed961defc 100644
--- a/src/subs.rs
+++ b/src/subs.rs
@@ -108,6 +108,11 @@ impl OptVariable {
             Some(Variable(self.0))
         }
     }
+
+impl fmt::Debug for OptVariable {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.clone().into_variable().fmt(f)
+    }
 }
 
 impl Into<Option<Variable>> for OptVariable {