diff --git a/src/debug.js b/src/debug.js
index 77e6384a..c5417f72 100644
--- a/src/debug.js
+++ b/src/debug.js
@@ -221,5 +221,6 @@ function enabled(name) {
 
 function coerce(val) {
   if (val instanceof Error) return val.stack || val.message;
+  if (typeof val === 'function') return val();
   return val;
 }