1 files changed, 11 insertions, 0 deletions
diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
index d1ec9b566d66..b9f3e6551de1 100644
--- a/src/libexpr/value-to-json.cc
+++ b/src/libexpr/value-to-json.cc
@@ -80,10 +80,21 @@ void printValueAsJSON(EvalState & state, bool strict,
             break;
         }
 
+	case tExternal:
+            v.external->printValueAsJSON(state, strict, str, context);
+            break;
+
         default:
             throw TypeError(format("cannot convert %1% to JSON") % showType(v));
     }
 }
 
 
+void ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
+      std::ostream & str, PathSet & context)
+{
+    throw TypeError(format("cannot convert %1% to JSON") % showType());
+}
+
+
 }