diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index efdad18e1..1fecd1c97 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -352,7 +352,7 @@ Derivation parseDerivation(
         expect(str, "erive(");
         version = DerivationATermVersion::Traditional;
         break;
-    case 'r':
+    case 'r': {
         expect(str, "rvWithVersion(");
         auto versionS = parseString(str);
         if (versionS == "xp-dyn-drv") {
@@ -365,6 +365,9 @@ Derivation parseDerivation(
         expect(str, ",");
         break;
     }
+    default:
+        throw Error("derivation does not start with 'Derive' or 'DrvWithVersion'");
+    }
 
     /* Parse the list of outputs. */
     expect(str, "[");