diff --git a/src/libexpr/gc.cc b/src/libexpr/gc.cc
index 8320c64ee..902a4c712 100644
--- a/src/libexpr/gc.cc
+++ b/src/libexpr/gc.cc
@@ -295,8 +295,10 @@ std::pair<Size, Size> GC::Arena::freeUnmarked()
                 obj->unmark();
             } else {
                 //debug("FREE OBJECT %x %d %d", obj, obj->type, objSize);
+                #if GC_DEBUG
                 for (Size i = 0; i < objSize; ++i)
                     ((Word *) obj)[i] = 0xdeadc0dedeadbeefULL;
+                #endif
                 objectsFreed += 1;
                 wordsFreed += objSize;
                 if (curFree) {
@@ -333,14 +335,6 @@ bool GC::isObject(void * p)
     return false;
 }
 
-void GC::assertObject(void * p)
-{
-    if (!isObject(p)) {
-        printError("object %p is not an object", p);
-        abort();
-    }
-}
-
 GC::ArenaList::ArenaList()
 {
     static Size initialHeapSize = std::stol(getEnv("GC_INITIAL_HEAP_SIZE", "1000000")) / WORD_SIZE;
diff --git a/src/libexpr/gc.hh b/src/libexpr/gc.hh
index a0ca3f307..dfd8c1b08 100644
--- a/src/libexpr/gc.hh
+++ b/src/libexpr/gc.hh
@@ -277,7 +277,15 @@ public:
 
     bool isObject(void * p);
 
-    void assertObject(void * p);
+    void assertObject(void * p)
+    {
+        #if GC_DEBUG
+        if (!isObject(p)) {
+            printError("object %p is not an object", p);
+            abort();
+        }
+        #endif
+    }
 };
 
 extern GC gc;