diff --git a/src/lstm/network.h b/src/lstm/network.h
index 0a62baf29b..feb1206f58 100644
--- a/src/lstm/network.h
+++ b/src/lstm/network.h
@@ -140,9 +140,7 @@ class TESS_API Network {
   const std::string &name() const {
     return name_;
   }
-  virtual std::string spec() const {
-    return "?";
-  }
+  virtual std::string spec() const = 0;
   bool TestFlag(NetworkFlags flag) const {
     return (network_flags_ & flag) != 0;
   }