diff --git a/src/avian/classpath-common.h b/src/avian/classpath-common.h
index 1137f9d2c9..44f8cf5491 100644
--- a/src/avian/classpath-common.h
+++ b/src/avian/classpath-common.h
@@ -266,8 +266,13 @@ clone(Thread* t, object o)
            reinterpret_cast<void**>(o) + 1,
            size - BytesPerWord);
   } else {
+    object classNameSlash = className(t, objectClass(t, o));
+    PROTECT(t, classNameSlash);
+    THREAD_RUNTIME_ARRAY(t, char, classNameDot, byteArrayLength(t, classNameSlash));
+    replace('/', '.', RUNTIME_ARRAY_BODY(classNameDot),
+            reinterpret_cast<char*>(&byteArrayBody(t, classNameSlash, 0)));
     throwNew(t, Machine::CloneNotSupportedExceptionType, "%s",
-             &byteArrayBody(t, className(t, objectClass(t, o)), 0)); 
+             RUNTIME_ARRAY_BODY(classNameDot));
   }
 
   return clone;
diff --git a/src/compile.cpp b/src/compile.cpp
index 649e55fb5f..c7c1b7ad59 100644
--- a/src/compile.cpp
+++ b/src/compile.cpp
@@ -3034,10 +3034,19 @@ void
 checkCast(MyThread* t, object class_, object o)
 {
   if (UNLIKELY(o and not isAssignableFrom(t, class_, objectClass(t, o)))) {
+    object classNameFrom = className(t, objectClass(t, o));
+    object classNameTo   = className(t, class_);
+    PROTECT(t, classNameFrom);
+    PROTECT(t, classNameTo);
+    THREAD_RUNTIME_ARRAY(t, char, classFrom, byteArrayLength(t, classNameFrom));
+    THREAD_RUNTIME_ARRAY(t, char, classTo,   byteArrayLength(t, classNameTo));
+    replace('/', '.', RUNTIME_ARRAY_BODY(classFrom),
+            reinterpret_cast<char*>(&byteArrayBody(t, classNameFrom, 0)));
+    replace('/', '.', RUNTIME_ARRAY_BODY(classTo),
+            reinterpret_cast<char*>(&byteArrayBody(t, classNameTo,   0)));
     throwNew
-      (t, Machine::ClassCastExceptionType, "%s as %s",
-       &byteArrayBody(t, className(t, objectClass(t, o)), 0),
-       &byteArrayBody(t, className(t, class_), 0));
+      (t, Machine::ClassCastExceptionType, "%s cannot be cast to %s",
+       RUNTIME_ARRAY_BODY(classFrom), RUNTIME_ARRAY_BODY(classTo));
   }
 }