diff --git a/lib/chai-as-promised.js b/lib/chai-as-promised.js
index 0b84194..a10f892 100644
--- a/lib/chai-as-promised.js
+++ b/lib/chai-as-promised.js
@@ -77,7 +77,17 @@ module.exports = (chai, utils) => {
     }
 
     function getReasonName(reason) {
-        return reason instanceof Error ? reason.toString() : checkError.getConstructorName(reason);
+        const constructorName = checkError.getConstructorName(reason);
+        const message = checkError.getMessage(reason);
+
+        if (reason instanceof Error) {
+            return reason.toString();
+        } else if (constructorName instanceof String) {
+            return constructorName;
+        } else if (message instanceof String) {
+            return message;
+        }
+        return reason;
     }
 
     // Grab these first, before we modify `Assertion.prototype`.