diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 2e57c1708..cd4a2df6f 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -182,7 +182,19 @@ Goal::Co DerivationGoal::haveDerivation()
                 }
             }
 
-            assert(success.builtOutputs.count(wantedOutput) > 0);
+            /* If the wanted output is not in builtOutputs (e.g., because it
+               was already valid and therefore not re-registered), we need to
+               add it ourselves to ensure we return the correct information. */
+            if (success.builtOutputs.count(wantedOutput) == 0) {
+                debug(
+                    "BUG! wanted output '%s' not in builtOutputs, working around by adding it manually", wantedOutput);
+                auto realisation = assertPathValidity();
+                realisation.id = DrvOutput{
+                    .drvHash = outputHash,
+                    .outputName = wantedOutput,
+                };
+                success.builtOutputs.emplace(wantedOutput, std::move(realisation));
+            }
         }
     }