diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index 74fafd4266c..d003f8ec684 100755
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -521,9 +521,11 @@ static void main_nix_build(int argc, char * * argv)
             if (counter)
                 drvPrefix += fmt("-%d", counter + 1);
 
-            auto builtOutputs = store->queryDerivationOutputMap(drvPath);
+            auto builtOutputs = store->queryPartialDerivationOutputMap(drvPath);
 
-            auto outputPath = builtOutputs.at(outputName);
+            auto maybeOutputPath = builtOutputs.at(outputName);
+            assert(maybeOutputPath);
+            auto outputPath = *maybeOutputPath;
 
             if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>()) {
                 std::string symlink = drvPrefix;